bumper humping
When a vehicle behind you is so close to your bumper that it could hump it!
That person behind me is bumper humping me.
hump bumper
The pads they put on the walls in hotel rooms over the beds instead of headboards, so while you're nailing a chick her head doesn't bang the wall.
I was screwing her so hard that night she kept smacking her head on the hump bumper.