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The muscle developed in the forearm from jerking off too much. Often exaggerated amongst friends to make one appear to jerk off more than they might.
Mike: Dude, you been going to the gym yet?
Nate: I started going a week ago.
Shannon: Don't listen to him, we broke up a week ago and that's just his jerk muscle gone crazy.
by Skrait Buttah July 29, 2006