When cuddling, the useless arm that gets stuck in between the two cuddlers due to the lack of a better place to put it. Usually ends up by the crotch, intentionally or not.
"Are you trying to grope me?!"
"No, it's just gimp arm."
Someone's arm that is unable to fully extend or function
Guy 1: "Hey, why is Chris shooting with his left hand? I thought he was right handed."
Guy 2: "Yeah, he has a gimp arm, it can't fully extend."