When pigs fly means never. Pigs can't fly, and that's the whole joke. People say it sarcastically, usually as a short reply to a promise or hope they don't believe: “Will he pay the money back?” “Yeah, when pigs fly.”
When would you say when pigs fly?
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Yeah, when pigs fly. (sarcastic reply); will + verb + when pigs fly; Pigs might fly. (UK)
- 'Do you think Tom will arrive on time tonight?' 'Yeah, when pigs fly.'
- My brother says he'll start jogging every morning before work. When pigs fly!
- The landlord will fix the heating when pigs fly, so I bought an electric heater.
What changed
First versionHe'll clean his room when pigs will fly.
RevisionHe'll clean his room when pigs fly.
The pigs would fly in the future, so will can seem logical. But after when, English uses the present tense for the future: when pigs fly, when I finish, when she arrives. So will fly was replaced with fly.
In Britain the same joke is more often built differently: Pigs might fly. It's the reply to a promise that's too bold to believe: 'I'll be on time tomorrow.' 'Yeah, and pigs might fly.' When pigs fly is the American version, but it's understood everywhere.
Your turn
'Will your boss give everyone a pay rise this year?' 'Sure, ___. He's never done it before.'
Show the answer
when pigs fly — The boss has never given a pay rise, so sure sounds sarcastic and means never: when pigs fly. Once in a blue moon means very rarely, not never, and it isn't used as a sarcastic reply to a question.
Write a short dialogue: someone you know makes a promise, and you don't believe it. End your reply with when pigs fly.
'I'll stop buying new shoes this year,' my sister said. 'Sure, when pigs fly.'
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