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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
 
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