Noun, the slimy substance that seeps out from between the fingers of a closed fist after a person has ejaculated in their own hand.
Jack used a tissue to wipe the knuckle butter off of his hand after he jacked off.
by Bentrok April 28, 2006
When you fist a girls asshole then switch to her pussy and the butt/pussy juices mix to create a tan frothy cream.
"After I fisted my girls butt i switched to da pussy, my wrist was drippin with knuckle butter son!"
by IAbmx May 12, 2015