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

People say "When pigs fly" to describe the impossible.

From Science Daily

It seemed for a while that California’s controversial pork law would take effect only when pigs fly.

From Los Angeles Times

"Lukas kept saying that he was going to come to Poland, but always chose to play elsewhere, and it gradually became a joke. 'When Podolski signs for Gornik' meant something like 'when pigs fly' in Silesia - that was never going to happen," Wirtualna Polska journalist Piotr Kozminski told BBC Sport.

From BBC

“For me this bill has a when pigs fly sort of quality to it,” said Democratic Rep. Omari Hardy.

From Washington Times

But when pigs fly — that's something unheard of, even by Taiwan's standards.

From Washington Post