To break ones will. To make one conform. Ass rape. To punk. The term "stump" represents the hieghth of the act taking place. To break someone over a stump and rape them. Hence the term broke.
When some bastard in the throes of bestiality trains a horse, mule, or any livestock animal into not walking away from a tree stump whenever he, ahem... "assumes the postion"...
if said bastard gets his freak on with said beast of "burden", and the animal has not tried to run off, it's stumpbroke.
"That skank-ass hoochiemama is just like a stumpbroke goat, she's spent her whole life backin up to somethin doggystyle"
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.