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.
"Danm! Someone tell that knucklebutt bitch to get some meat on her!"
by Joe July 27, 2003
To slip one's knuckle into the crack of an unsuspecting victim. Similar to a credit card but using the knuckle.
When Sally bent down to pick up her shoes, Bryan reach over and gave her the 'ole knuckle butt.
by akellz April 17, 2012