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 major knuckle-Ass"
It is said of the situation where a person has the bad luck to make contact with his testicles against an undefined surface or object, intentioned or not.
Given the nature of the word, it is more appropriate to design cases where the interaction is made with a moving object, for example, a ball.
Although it is extremely painful for the victim, it tends to be considerably funny to people who witness it.
Today in the baseball game the pitcher took a nutshot; the baseball hit him in the nuts.
Man, I just watched the funniest nutshot video ever.