Stump Grindin' occurs when a man and a woman have sex in the missionary position. The man has his legs cut off at the knee. This poses an awkward thrusting resembling that of midget porn in fast forward.
Jimmy: "Wow, did you hear about what happened last night with Tina?"
Geoffrey: "No, what?"
Jimmy: "She was stump grindin' with that amputee."
Geoffrey: "OH SHI-"