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1.
A condition where a person is completely normal/nice from day to day life, and a "monster" or goon while on the ice/field/etc.
Ryan: Hey, you know Zach?
Ben: Ya he's a cool guy.

Ryan: Did you hear he beat a guy up in the game last night?
Ben: Seriously?!?
Ryan: Ya I know, he suffers from a bad case of Jekyll and Hyde Syndrome
by Diagnosis #1 May 14, 2011