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2.
Idiomatic expression used to describe an event or scenario that is so unlikely that it will likely never occur.
Q: Dude, I'm tired of the beach. Wanna go and ride that fake, plastic wave that they setup at Wal-Mart?
A: Yeah!
Q: Right on! Let's roll!
A: Hold on, I'd rather do that when hell freezes over. You stupid nincompoop.
by broceanographer July 20, 2006
 
1.
signifies that something will never, ever happen.
1: so when will the Bengals win the Super Bowl?
2: when hell freezes over, that's when.
by KRHimself November 29, 2004