pigs might fly

Từ điển Oxford Advanced Learner 8th

pigs might ˈfly idiom
(BrE) (NAmE when pigs ˈfly) (ironic, saying)used to show that you do not believe sth will ever happen
•‘With a bit of luck, we'll be finished by the end of the year.’ ‘Yes, and pigs might fly!’
Main entry: ↑pig_idiom