It's a butt that is is very big and attractive were you wanna hit it. Like in bumper cars you wanna hit the other people.
My boyfriend has a bumper butt
by Gpnfa July 10, 2014
A game played while on the bus whereby two (or more) people repeatedly touch butts (tap ass, moon landing, etc).
by gjcourt November 9, 2012
A Butt Bumper is a guy that screws around. He is a bumper of butts, Also used to describe a gay guy. Like hey look at the Butt Bumper over there talking to his boy friend, Butt Bumper. Also meaning a person that procrastinates Example hey dude you need to hire someone else this dude is a serious Butt Bumper. Also A Butt Head
Butt Bumper,
by Daman Money January 25, 2010