Meaning
Used sarcastically to say that something is completely impossible and will never happen.
It’s typically a dismissive or humorous response to a suggestion or promise the speaker considers wildly unrealistic.
Origin
The phrase is often said to derive from an old Scottish proverb, though the precise date and documentation of that claimed origin are not firmly established. What is better documented is a long literary history of winged-pig imagery used for absurd impossibility: François Rabelais’ “Fourth Book of Gargantua and Pantagruel” (1553) already describes a fantastical winged swine, and Lewis Carroll’s “Alice’s Adventures in Wonderland” (1865) has the Duchess tell Alice she has “as much right” to think as “pigs have to fly,” with a related aside about pigs having wings recurring in “Through the Looking-Glass” as well. The image’s persistence across centuries of English literature likely helped fix “pigs fly” as the default absurd-impossibility image in the language, whatever its exact folk-proverb origins.
Examples
He’ll clean his room when pigs fly.
She’ll apologize when pigs fly.
They’ll agree to that deal when pigs fly.
A: Do you think he’ll ever admit he’s wrong? B: Sure, when pigs fly.
Frequently asked questions
What does 'When pigs fly' mean?
Used sarcastically to say that something is completely impossible and will never happen.
It’s typically a dismissive or humorous response to a suggestion or promise the speaker considers wildly unrealistic.