(idiomatic, vulgar) Usually used as a response (often with hint of sarcasm) to a situation that you think there is no chance of ever occurring.
‘I am sure he'll pay you back tomorrow.’ ‘Yeah right, and monkeys might fly out of my butt.’
Origin of monkeys-might-fly-out-of-my-butt
Popularized by the 1992 film Wayne's World. Likely a variation on pigs might fly.