a cum sloth is one who moves only for the sake of consuming cum. he/she loves cum in their mouths 24/7
guy1: dude she wouldn't leave her dorm until i told her i wanted head, then she came over and sucked my cock for an hour.
guy2: wow, what a cum sloth!!!
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.