monkeys might fly out of my butt

Popularized by the 1992 film Wayne's World. Likely a variation on pigs might fly.

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monkeys might fly out of my butt

1. (Canada, US, slang, vulgar) A response to a suggested situation that one thinks has no chance of ever occurring.

   ‘I am sure he'll pay you back tomorrow.’ ‘Yeah right, and monkeys might fly out of my butt.’

• pigs might fly