Noun. A bongobum is a backside which has relative features to a bongo drum. For example, the buttocks can have a likeliness sound when played as an instrument, a visual likeliness and a likeliness feeling to a bongo drum.
Boy: Woah! When I hit that girls booty it sounded like bongos!
Boy's friend: Yeh! That girl has such a bongobum!