When pigs fly
/wɛn pɪɡz flaɪ/informal, humorousUK · US · AU · IE · CA
What does “When pigs fly” mean?
When pigs fly means this expression means that an event will never happen because it is completely impossible.
This phrase describes an event that will never take place. Because real pigs cannot fly, using this picture shows that an idea is completely unrealistic.
The tone is playful, lighthearted, and often a little sarcastic. It is best used in casual conversations with friends rather than in formal situations.
When do people say it?
People say this to show strong disbelief or to reject a request playfully.
“He promised to clean his bedroom today, but that will happen when pigs fly.”
Is it formal or informal?
Register: informal, humorous. It is most at home in everyday conversation rather than formal writing. Most common in UK and US and AU and IE and CA.
Where does it come from?
An old Scottish proverb from the late 1500s mentioned flying pigs to describe impossible things. The image became much more famous after Lewis Carroll used it in Alice's Adventures in Wonderland in 1865.
Origin confidence: Well documented
How do you say “When pigs fly” in other languages?
Last reviewed 2026-09-11