When pigs fly is a cheerful way to say never. Pigs have no wings, so the day they fly is the day the impossible thing happens.
The idiom is a reply, not a statement. It answers a plan, a promise or a suggestion that the speaker thinks is hopeless.
This page explains the meaning, the grammar, the register and the older forms of the saying. The 10-question drill at the end explains every answer.
What it means
In one sentence: When pigs fly means never: it says that something is impossible or will definitely not happen.
What the idiom means in everyday English
- It means never: "He will pay you back when pigs fly."
- It usually answers an idea that the speaker thinks is unrealistic.
- The tone is humorous and often friendly, though it can sting if the dream was serious.
- It can describe a person’s behaviour or an event: "She will tidy her room when pigs fly."
How the idiom behaves in a sentence
- The phrase often follows a clause with will: "I will sing on stage when pigs fly."
- It can stand alone as a reply: "When pigs fly!"
- The verb in the main clause does not change the idiom; pigs fly stays fixed.
- Never add a before pigs: when pigs fly, not when the pigs fly.
When the idiom does not fit
- Do not use it for something merely unlikely; for that, say "it is a long shot".
- It can sound dismissive, so avoid it when someone is sharing a serious hope.
- Do not use it in formal writing; say "that is not going to happen".
- It is not about farming or animals, of course; the picture is the joke.
Where it comes from
The idea is old. Scottish and English collections of proverbs from the 1600s already have a close form, "when the swine fly". The modern wording with pigs is later, and the image has stayed the same: a flying pig stands for something impossible.
Use it or not
Register: Informal and humorous. When pigs fly is fine in conversation between friends and in light writing. It can be sarcastic, so check the mood before using it about someone’s plan. In formal writing, use "that is not going to happen" or "that is impossible".
Examples
- He will clean the garage when pigs fly.The idiom says the cleaning will never happen.
- When pigs fly she will admit she was wrong.The phrase can come first for emphasis.
- A: Will you lend him your car? B: When pigs fly!The idiom works alone as a short reply.
- The company will reduce prices when pigs fly.A humorous prediction of an impossible change.
- They say the train will arrive on time when pigs fly.The idiom mocks a common complaint.
- I will run a marathon when pigs fly.A light way to refuse a personal challenge.
- He promised to cook dinner, and that will happen when pigs fly.The idiom comments on a promise the speaker doubts.
Similar idioms
when hell freezes over
never, stronger and less polite than when pigs fly.
not in a million years
never, a common and direct phrase.
on the twelfth of never
never, old-fashioned and playful.
fat chance
almost no chance, sarcastic.
that ship has sailed
the chance is gone, different from never happening.
Practice drill
Answer all 10 questions. Each answer is marked at once, and the meaning is explained under the question.
Tom will pay back the money ___.
Show answer and explanation
Answer: A. when pigs fly When pigs fly means never, which fits the speaker saying Tom will not pay the money back.
Which sentence uses when pigs fly correctly?
Show answer and explanation
Answer: B. She will sell her motorbike when pigs fly; she loves it. The idiom means never, so saying she will never sell the bike fits; a real flying pig is literal.
What does when pigs fly mean?
Show answer and explanation
Answer: C. never When pigs fly means something will never happen, because pigs cannot fly.
Your friend asks if you will ever get up at 5 a.m. to run. You hate mornings. What can you say?
Show answer and explanation
Answer: D. When pigs fly. When pigs fly is the fixed idiom for never, which matches your friend’s impossible idea.
Which idea fits the idiom as a reply?
Show answer and explanation
Answer: A. a promise that will never be kept The idiom answers a suggestion the speaker believes will never come true, such as an empty promise.
Choose the sentence with the same meaning: "He will apologise when pigs fly."
Show answer and explanation
Answer: B. He will never apologise. When pigs fly means never, so the sentence means he will never apologise.
Which is correct?
Show answer and explanation
Answer: C. I will lend him money when pigs fly. The fixed phrase is when pigs fly, with no article and the plural pigs.
A colleague suggests cancelling the holiday at the last minute. You think it will never happen. What can you say?
Show answer and explanation
Answer: D. That will happen when pigs fly. When pigs fly fits a suggestion you believe will never happen, and it keeps the fixed form with no article.
Which sentence does NOT use the idiom correctly?
Show answer and explanation
Answer: A. The pigs fly over the farm every morning. The last sentence is about real, flying animals, while the idiom means never.
A friend says, "I will become an astronaut when pigs fly." What does the friend mean?
Show answer and explanation
Answer: B. The friend believes it will never happen. When pigs fly means never, so the friend expects the astronaut plan will not come true.
Questions learners ask
Does when pigs fly mean unlikely or impossible?
Impossible, or so unlikely that it may as well be impossible. The phrase is stronger than "a long shot", which allows a small chance.
Is when pigs fly rude?
It can be. The phrase is humorous, but it dismisses the other person’s idea. Between friends it is a joke. In a serious conversation about someone’s hope, use gentler words.
Why pigs?
Pigs cannot fly, so they are a funny image of impossibility. Scottish and English proverb collections from the 1600s already use a close form, "when the swine fly", and the phrase has kept the same joke ever since.
More idioms
Up in the air
If something is up in the air, it is not decided yet and the final answer is uncertain.
The ball is in your court
If the ball is in your court, it is your turn to take action or make the next decision.
Cut corners
To cut corners is to do a job in a quicker or cheaper way by skipping the proper steps, usually with poor results.
Browse all the idiom pages or take the free A1 to C2 level test to find your starting point.