when pigs fly

when pigs fly idiom
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.

The American Heritage® Dictionary of Idioms by Christine Ammer. Copyright © 2003, 1997 by The Christine Ammer 1992 Trust. Published by Houghton Mifflin Harcourt Publishing Company. All rights reserved.

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