when pigs fly

Idioms  

1. Never, as in Sure he'll pay for the drinks—when pigs fly. Equating the flight of pigs with something impossible dates from the early 1600s, when several writers alleged that pigs fly with their tails forward. The idiom is also put as pigs may fly.

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Example Sentences

However, his idea of a metropolitan police force - saddling Cedar Falls with Waterloo’s problems - will happen when pigs fly.

From Washington Times • Mar. 19, 2018

It’s been so long in Denver — high-school freshmen have never seen the Broncos win a title — one wondered if the Broncos would win another Super Bowl when pigs fly.

From Seattle Times • Jan. 28, 2014

"That will happen when pigs fly," said mac an Da'v, "and," said he, "I did not make any bargain with any king that is in the world."

From Irish Fairy Tales by Stephens, James