someone who does nothing. Sits around all day with their thumb up there ass to the knuckle.
Paul: Hey Todd, did you get that work done that you were supposed to do?
Todd: Aww, meant to, but didn't get around to it yet, sorry dude.
Paul: Yur a fuckin shit knuckle Todd, get to work!
A bump protruding from a log of shit resembling a knuckle. Shit knuckle.
That dude is a real shit knuckle.