Butt-knuckle means: 1. The smelly brown fingernail or thumb that is inserted into someone’s butt. The person who inserts the thumb or finger. It is the act of inserting a thumb or finger in one’s stomach. This could be due to boredom, or lack of interest. 4. Someone who complains often but doesn’t do anything or offers suggestions on how to fix the problem. (in Community Dictionary, added by Nicole Robinson)