When Pigs Fly

The phrase "when pigs fly" (alternatively, "pigs might fly") is an adynaton—a figure of speech so hyperbolic that it describes an impossibility. The implication of such a phrase is that the circumstances in question (the adynaton, and the circumstances to which the adynaton is being applied) will never occur. The phrase has been used in various forms since the 1600s as a sarcastic remark.