Immune to the laws of physics and incapable of independent activity, the butthumper finds your rear bumper on an open stretch of highway, and stays there. The asshole may be using you as a shield against radar, or as a fuel-mileage aid, but usually he/she is talking on the phone and relying on you to scan the road for them.

when you're hungover as shit after a spending a night drunk as a skunk

a person who uses there butt to hump the grass

Butthumper is a good term to call a homosexual man, who likes to take it in the end, who is being an asshole!

Butthumper, someone who is a homosexual faggot, and likes to take it in the end. Also someone who walks with their butt sticking in the air!