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.
A guy who is not gay but loves it when a girl penetrates his anus with her finger during sex.
Jeremy's motto is "your not gay if a girl is doing it to you. Guess you can say he's a knuckle butt.