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.
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
Dorothy: Whoa, look at your arm muscle! You've been working out, huh?
Bob: ummm... it's my jerk-off muscle...
Bob: ummm... it's my jerk-off muscle...
by Joedaman March 19, 2010