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).
"Wow that was an exhilterating game of bumper butts! I feel like I owe you a drink!"
by gjcourt November 09, 2012