when pigs fly
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® Idioms Dictionary Copyright © 2002, 2001, 1995 by Houghton Mifflin Harcourt Publishing Company. Published by Houghton Mifflin Harcourt Publishing Company.