When pigs fly is a mocking way to say something will never happen. Yes, when pigs fly, that will happen when pigs fly, sure, when pigs fly.
It is a flat refusal dressed as agreement, which is what makes it sting. Other languages do the same with different animals, and English also has the version about a cold day somewhere very hot.
1phrasewhen pigs flyA mocking way to say something will never happen.
“He says he'll pay me back next week, but I'll believe that when pigs fly.”
when pigs fly is a refusal.
From the plain impossibility of a pig flying, a form of the ancient joke about impossible conditions.
Referencing the fact that pigs do not have wings and cannot fly.
Etymology from Wiktionary, CC BY-SA
Practised in Idioms