A Bubble Yum Bum is American slang for a large buttocks. This slang term has recently become more popular since the release of the American Rapper Flo-Rida’s song “Can’t Believe It” ft. Pitbull. It is fairly common in American slang for a large buttocks to be described as a “bubble”. Bubble Yum is a brand of American bubble gum.
Bubble Yum Bum has not been a popular slang term in recent years, so it will be interesting to see if this term catches on.