A Montana stump grinder is a sexual act. It begins with a man and a woman, or a man and a man, and the giver takes rough grit sand paper, rolls it into a tube big enough for the receivers penis to fit through, and stimulates the penis with it, similar to a hand job.
"You would not believe how sore I am from the Montana stump grinder I got last night. It was the best I've ever had!"
"Ask your lover for a Montana stump grinder if you like to party."