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1.
A theory that subscribes to the notion that there are only 2 outcomes to any kind of situation. It either will happen, or it won't, no matter how unrealistic. There is therefore 50% chance either way.

Also used to piss someone off, in an un-winnable argument.
Anybody: Oi, Gimp, Scunthorpe Utd will win the Champions League within 3 years.

The Gimp: No they won't, Man Utd will.

Anybody: Fuck off Gimp. 50/50 theory says there is a 50% chance they will. Its simple, they either will or they won't, there are only 2 possible outcomes so fuck off!
by Steeman May 07, 2005