The result of an improper angle of approach when wiping your ass from a particularly messy poop-session. Poop gets on your knuckle. Usually occurs when you lift a cheek to the side and attempt a wipe.
Steve liked to lean to the left when he wiped his ass. This often resulted in a nasty case of poop knuckle.
The second knuckle in (heading towards the hand). Originating from the knuckle at which you stop at when inserting a digit into another's asshole.
Used to warm up/stretch out an asshole before anal sex.