A very nice ass. Doesn't necessarily have to be huge, in fact, they're normally not. It's just a tasty looking bum-bum.
John: That slut sure knows how to make a guy feel good.
Eugene: Ohhh yeah. How about that bubblebum?
When someone chews gum with their ass and makes a bubble thus gives it the name bubblebum
that bubblebum you blew was fantastic