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Idioms are fun phrases that mean something different than the actual words inside them. People use them every day to make speaking and writing much more colorful and exciting. In this article, you will learn fifty special expressions connected to people who are untrustworthy and cannot be relied upon.
In this lesson, we’ll look at idioms that mean being dishonest or breaking a promise. Let’s get started and explore some of these helpful expressions.
Idioms for Untrustworthy
1. Snake in the grass
Definition: A secret and deceitful enemy who pretends to be a friend.
Used in a Sentence: We later found out that the quiet boy in our group was a snake in the grass who stole our ideas. / Do not trust him with your secrets because he acts like a snake in the grass.
Similar Expressions: Two faced person / Deceitful friend
2. Two-faced
Definition: Saying one thing to your face and saying bad things behind your back.
Used in a Sentence: Sarah realized her classmate was two-faced when she heard her spreading rumors. / You cannot rely on a two-faced friend who changes their attitude constantly.
Similar Expressions: Insincere / Deceptive
3. Wolf in sheep’s clothing
Definition: A dangerous or dishonest person who looks harmless on the outside.
Used in a Sentence: The charming salesman turned out to be a wolf in sheep’s clothing trying to cheat people. / Be careful who you let into your club, as some might be a wolf in sheep’s clothing.
Similar Expressions: Hidden threat / Dangerous impostor
4. Fair-weather friend
Definition: Someone who is your friend only when things are good and leaves when trouble starts.
Used in a Sentence: When Mark lost his soccer position, his fair-weather friend stopped talking to him. / You learn who your true companions are and who is just a fair-weather friend.
Similar Expressions: Unreliable pal / Selfish friend
5. Speak with a fork tongue
Definition: To tell lies or say one thing while meaning the opposite to trick people.
Used in a Sentence: The dishonest trader spoke with a fork tongue about the quality of the toys. / Never believe a person who is known to speak with a fork tongue.
Similar Expressions: Tell lies / Be deceitful
6. Sell out
Definition: To betray your principles, friends, or team for personal gain.
Used in a Sentence: Liam refused to sell out his teammates by sharing their game strategy. / It feels terrible when a trusted partner decides to sell out for a prize.
Similar Expressions: Betray others / Turn traitor
7. Bite the hand that feeds you
Definition: To harm or act dishonestly toward someone who helps and supports you.
Used in a Sentence: Stealing money from the kind shopkeeper who trained you is biting the hand that feeds you. / Do not bite the hand that feeds you when your parents give you advice.
Similar Expressions: Be ungrateful / Betray a helper
8. Play both sides
Definition: Supporting two opposing groups dishonestly to benefit yourself.
Used in a Sentence: The untrustworthy politician tried to play both sides during the community debate. / You will lose everyone’s respect if you keep trying to play both sides.
Similar Expressions: Be deceitful / Act treacherously
9. False as a dice
Definition: Completely dishonest and impossible to trust in any situation.
Used in a Sentence: His promise to return the borrowed game was as false as a dice. / Do not lend your bicycle to someone whose word is as false as a dice.
Similar Expressions: Untrustworthy / Fully dishonest
10. Sell someone down the river
Definition: To betray someone who trusts you in order to save yourself.
Used in a Sentence: He felt deep guilt after trying to sell his project partner down the river. / A true friend would never sell you down the river when trouble comes.
Similar Expressions: Betray trust / Stab in the back
11. Ride for a fall
Definition: Acting in a dishonest or careless way that will eventually lead to ruin.
Used in a Sentence: Cheating on every test means he is just riding for a fall. / The dishonest store owner was riding for a fall from the very beginning.
Similar Expressions: Head for trouble / Asking for disaster
12. Not playing with a full deck
Definition: Describing someone whose judgment or honesty is deeply questionable.
Used in a Sentence: Believing his crazy promises is like trusting someone who is not playing with a full deck. / The shady stranger acted like he was not playing with a full deck.
Similar Expressions: Untreliable judgment / Questionable mind
13. Double dealer
Definition: A person who tricks others by acting dishonestly and deceitfully.
Used in a Sentence: We discovered that the ticket seller was a double dealer who took extra cash. / Avoid doing business with a known double dealer.
Similar Expressions: Cheat / Fraud
14. Smooth talker
Definition: Someone who uses charming words to trick people into trusting them.
Used in a Sentence: The smooth talker convinced us to buy broken toys with fake gold paint. / Do not let a smooth talker persuade you to hand over your allowance.
Similar Expressions: Charming liar / Con artist
15. Painted sepulchre
Definition: Looking good on the outside while being completely dishonest inside.
Used in a Sentence: His polite manners hid a painted sepulchre of mean intentions. / We learned that her sweet smile was just a painted sepulchre.
Similar Expressions: Fake exterior / Deceptive appearance
16. Cross your fingers behind your back
Definition: Making a promise while secretly planning to break it.
Used in a Sentence: He promised to share the snacks, but he crossed his fingers behind his back. / A real promise is never made with your fingers crossed behind your back.
Similar Expressions: Make a false promise / Lie secretly
17. Shady character
Definition: A suspicious and untrustworthy person who looks like they are hiding something.
Used in a Sentence: A shady character was lingering near the bicycle rack after school. / Mom told us never to talk to any shady character on the street.
Similar Expressions: Suspicious person / Untrustworthy individual
18. Sell a bill of goods
Definition: To trick someone into believing a false story or buying something fake.
Used in a Sentence: The scam artist tried to sell us a bill of goods about a magic pen. / Do not let anyone sell you a bill of goods regarding easy homework answers.
Similar Expressions: Deceive / Trick with lies
19. Pull the wool over someone’s eyes
Definition: To trick or deceive someone so they do not see the truth.
Used in a Sentence: The clever magician tried to pull the wool over our eyes with card tricks. / You cannot pull the wool over the teacher’s eyes regarding missing work.
Similar Expressions: Fool someone / Deceive completely
20. Playing possum
Definition: Pretending to be innocent or helpless to trick people around you.
Used in a Sentence: The mischievous puppy was playing possum when we checked the torn pillow. / Do not let him pretend he is playing possum when he broke the rule.
Similar Expressions: Fake innocence / Act deceitfully
21. Rotten to the core
Definition: Describes someone who is completely dishonest and lacks any good morals.
Used in a Sentence: The thief who stole from the charity box was rotten to the core. / We realized the bully was rotten to the core after he lied again.
Similar Expressions: Completely corrupt / Deeply dishonest
22. Speak out of both sides of your mouth
Definition: Saying conflicting things to different people to deceive everyone.
Used in a Sentence: The dishonest committee member kept speaking out of both sides of his mouth. / You lose credibility when you speak out of both sides of your mouth.
Similar Expressions: Be hypocritical / Lie consistently
23. Promise the moon
Definition: Making grand promises that you have no intention or ability to keep.
Used in a Sentence: The car salesman promised the moon, but the vehicle broke down instantly. / Do not trust friends who promise the moon but never help out.
Similar Expressions: Overpromise / Make empty vows
24. Backstabber
Definition: Someone who pretends to be your friend before betraying you secretly.
Used in a Sentence: Finding out she was a backstabber broke my trust completely. / Never share personal secrets with a known backstabber.
Similar Expressions: Traitor / Secret betrayer
25. Empty vessel
Definition: Someone who talks big but has no reliable character or honesty.
Used in a Sentence: His grand claims about winning the race turned out to be an empty vessel. / Do not rely on an empty vessel for honest advice.
Similar Expressions: All talk / Untrustworthy braggart
26. Slip through your fingers
Definition: Describing a promise or deal made by someone who is too slippery to trust.
Used in a Sentence: The dishonest worker’s agreement seemed to slip through our fingers. / Trying to pin down a liar is like watching water slip through your fingers.
Similar Expressions: Unreliable agreement / Slippery deal
27. Walk all over someone
Definition: To treat a trusting person dishonestly and take advantage of them.
Used in a Sentence: He is too kind, and people sometimes try to walk all over him. / A trustworthy friend will never walk all over your feelings.
Similar Expressions: Take advantage / Mistreat cruelly
28. False alarm
Definition: A warning or claim that turns out to be untrustworthy and untrue.
Used in a Sentence: The rumor about school closing early was just a false alarm. / Hearing the doorbell ring was a false alarm because nobody was there.
Similar Expressions: Untrue report / Fake warning
29. Throw under the bus
Definition: To sacrifice a friend or colleague to save yourself from trouble.
Used in a Sentence: He decided to throw his partner under the bus when the teacher asked questions. / A loyal teammate will never throw you under the bus.
Similar Expressions: Betray a partner / Sacrifice others
30. Play fast and loose
Definition: To act in a reckless, dishonest, and untrustworthy manner with rules.
Used in a Sentence: The game referee played fast and loose with the scoring rules. / You cannot trust someone who plays fast and loose with promises.
Similar Expressions: Act recklessly / Be unreliable
31. Blow hot and cold
Definition: Constantly changing your attitude and loyalty toward someone.
Used in a Sentence: It is hard to trust her because she blows hot and cold every day. / He blew hot and cold about joining our science club project.
Similar Expressions: Be inconsistent / Change constantly
32. Build castles in Spain
Definition: Making unrealistic, untrustworthy plans that will never happen.
Used in a Sentence: His promise to buy us a mansion was just building castles in Spain. / Stop building castles in Spain and focus on honest homework.
Similar Expressions: Unrealistic dreams / False hopes
33. Fair-weather pilot
Definition: Someone who helps only when things are easy but quits in a crisis.
Used in a Sentence: The team captain proved to be a fair-weather pilot during the championship match. / We need reliable friends, not a fair-weather pilot.
Similar Expressions: Unreliable leader / Quitters
34. Smoke and mirrors
Definition: Deceptive tricks used to disguise dishonesty and make things look better.
Used in a Sentence: The store’s discount sale turned out to be just smoke and mirrors. / Do not fall for his smoke and mirrors explanation of the broken vase.
Similar Expressions: Deception / False illusion
35. Feed someone a line
Definition: To tell someone a false story or a bunch of lies on purpose.
Used in a Sentence: He tried to feed us a line about why his homework was missing. / Do not let him feed you a line about his magical video game skills.
Similar Expressions: Tell fibs / Feed falsehoods
36. Lead someone down the garden path
Definition: To trick or deceive someone into believing a false promise.
Used in a Sentence: The dishonest card player tried to lead us down the garden path. / She led her parents down the garden path about cleaning her room.
Similar Expressions: Mislead / Deceive deliberately
37. Trick of the trade
Definition: A dishonest shortcut used by shady people to fool others.
Used in a Sentence: Using fake weights at the market was a dirty trick of the trade. / Honesty is better than any shady trick of the trade.
Similar Expressions: Deceptive method / Dishonest trick
38. Wooden nickel
Definition: Something fake or worthless given away by an untrustworthy person.
Used in a Sentence: His promise of free tickets turned out to be as worthless as a wooden nickel. / Do not accept a wooden nickel from someone you do not know.
Similar Expressions: Worthless promise / Fake item
39. Play the hypocrite
Definition: Pretending to be honest while doing dishonest things in secret.
Used in a Sentence: He likes to play the hypocrite by telling others not to lie while he cheats. / Stop playing the hypocrite and admit your mistake.
Similar Expressions: Be insincere / Act falsely
40. Keep an ear to the ground
Definition: Listening carefully to catch people who are spreading lies or acting shady.
Used in a Sentence: The principal kept an ear to the ground to catch dishonest behavior. / Keep an ear to the ground for any untrustworthy rumors.
Similar Expressions: Listen for clues / Watch out
41. Sell snake oil
Definition: To trick people into buying fake remedies or dishonest promises.
Used in a Sentence: The traveling merchant tried to sell snake oil to the villagers. / His claim that the toy could fly was just selling snake oil.
Similar Expressions: Peddle frauds / Sell lies
42. Cut corners
Definition: Doing a dishonest or lazy job to save effort while pretending it is fine.
Used in a Sentence: The builder cut corners on the treehouse roof, making it unsafe. / Never cut corners on safety rules during science experiments.
Similar Expressions: Do sloppy work / Be dishonest
43. Play fast with the truth
Definition: Changing facts slightly to deceive people without telling a direct lie.
Used in a Sentence: He liked to play fast with the truth when explaining why he was late. / Playing fast with the truth destroys people’s trust in you.
Similar Expressions: Stretch facts / Be deceitful
44. False friend
Definition: Someone who pretends to care about you but betrays your trust.
Used in a Sentence: Discovering she was a false friend hurt more than losing the game. / Choose your companions carefully to avoid a false friend.
Similar Expressions: Untrustworthy pal / Deceptive friend
45. Put on an act
Definition: Pretending to be honest and kind when you are actually deceitful.
Used in a Sentence: He put on an act of being helpful before stealing the last cookie. / We saw through his polite act immediately.
Similar Expressions: Fake behavior / Pretend sincerity
46. Spin a yarn
Definition: To tell a made-up, untrustworthy story that is not true.
Used in a Sentence: Grandpa loves to spin a yarn about his childhood adventures on the farm. / Do not let him spin a yarn about why his chores are undone.
Similar Expressions: Tell a story / Make up lies
47. Talk big
Definition: Bragging about things you can never do or delivering false promises.
Used in a Sentence: He always talks big about winning races, but he never practices. / Do not trust someone who talks big and delivers nothing.
Similar Expressions: Brag endlessly / Make empty claims
48. Put one over on
Definition: To successfully trick or deceive an unsuspecting person.
Used in a Sentence: The sneaky student tried to put one over on the substitute teacher. / You cannot put one over on mom because she always finds out.
Similar Expressions: Trick someone / Fool successfully
49. Smooth operator
Definition: A charming, untrustworthy person who manipulates situations easily.
Used in a Sentence: The clever boy acted like a smooth operator when dividing the candy. / Watch out for that smooth operator trying to trade broken toys.
Similar Expressions: Manipulator / Sly person
50. False coin
Definition: Fake character or dishonest behavior hidden behind a nice smile.
Used in a Sentence: His kind words turned out to be false coin when he insulted me. / Guard your trust against anyone offering false coin.
Similar Expressions: Insincere nature / Deceptive character
Exercise to Practice
- We later found out that the quiet boy in our group was a snake in the grass who stole our ideas.
- Sarah realized her classmate was two-faced when she heard her spreading nice rumors.
- The charming salesman turned out to be a wolf in sheep’s clothing trying to cheat people.
- When Mark lost his soccer position, his fair-weather friend stopped talking to him.
- The dishonest trader spoke with a fork tongue about the quality of the toys.
- Liam refused to sell out his teammates by sharing their game strategy.
- Stealing money from the kind shopkeeper who trained you is biting the hand that feeds you.
- The untrustworthy politician tried to play both sides during the community debate.
- His promise to return the borrowed game was as honest as gold.
- He felt deep guilt after trying to sell his project partner down the river.
- Cheating on every test means he is just riding for a fall.
- Believing his crazy promises is like trusting someone who is not playing with a full deck.
- We discovered that the ticket seller was a double dealer who took extra cash.
- The smooth talker convinced us to buy broken toys with fake gold paint.
- His polite manners hid a painted sepulchre of mean intentions.
Answer Key
- True
- False
- True
- True
- True
- True
- True
- True
- False
- True
- True
- True
- True
- True
- True
Conclusion
Learning these idioms helps you understand English much better and gives you fun new ways to talk about people who are dishonest or untrustworthy. You can use them when talking about stories, fairy tales, or everyday caution with your friends and family.
Try using one or two of these expressions in a conversation or a story this week. Not only will your writing sound more exciting, but you will also have fun practicing your new skills.
Frequently Asked Questions
What are some common idioms for untrustworthy?
Some common idioms include snake in the grass, two-faced, and wolf in sheep’s clothing. These expressions are often used in everyday conversation to talk about people who are dishonest or deceitful.
Why do we use idioms in English?
Idioms make our language much more colorful, expressive, and fun than using plain words. They help us share big ideas and feelings in creative ways that people easily recognize.
How can I learn these idioms faster?
You can learn idioms faster by using them in daily sentences, writing short stories, and practicing with a friend. Making flashcards with pictures can also help you remember what each phrase means.
Are idioms used in everyday conversation?
Yes, native English speakers use idioms all the time during casual chats, television shows, and books. Learning them helps you understand jokes, stories, and conversations much more naturally.
Can idioms have more than one meaning?
Most idioms have one main accepted meaning, but how people use them can vary slightly depending on the context. Reading example sentences helps you see the exact way each phrase works.
