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.
I was playing ping-pong and the ball hit off my poop-knuckle.
A grandmother who feels the effects of old age by being unable to control her bowel movements.
"Upon entering the bathroomone morning, I discovered that shit had been shit all over the bathroom. I soon realized that I was living with a Poop Nanny."