Something said when comparing to something equally unlikely to happen. A flying pig was featured in the Simpsons in the episode in which Lisa Simpson becomes a vegetarian.

C. M. Burns: "I'll make that donation... when pigs fly!! Ha ha"
(pig seen flying through the window in the background)
Smithers: "Will you be making that donation now, sir?"
C. M. Burns: "No, I think I'd still rather not."