To slip one's knuckle into the crack of an unsuspecting victim. Similar to a credit card but using the knuckle.
by akellz April 17, 2012
A girl who is so skinny, most likely from anorexia, that when she wears a bikini you can see the bones of her pelvis move around in her ass when she walks because she has almost no butt cheeks.
by Joe July 27, 2003
Jeremy's motto is "your not gay if a girl is doing it to you. Guess you can say he's a knuckle butt.
by Ballard757 January 29, 2011