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 somethindoggystyle"