The condition of having an asshole itch so intense and deep that it feels like the only way you can properly scratch it is to stick your finger up your ass all the way up to the knuckle, and scratch your asshole from the inside.
A case of knuckle ass might be the result of poor hygiene (see hanus anus) or could just be an unlucky ass itch that anyone can get from time to time.
"I took a shit this morning and I guess I didn't wipe well enough. Now I've got majorknuckle-Ass"
when you're holding up your phone and making faces at it, as though you are taking a selfie, but you're really taking a picture of the person across from you or the wall or anything else that seems interesting but you don't want to be caught dead taking a picture of.
This action is often made more convincing by wiggling the eyebrows or opening the mouth, to pretend you're trying to get a Snapchat filter to work.