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Wiktionary
bubble butt

n. 1 (context US slang LGBT English) Large, rounded buttocks. 2 (context US slang English) A fat person.

Wikipedia
Bubble Butt

"Bubble Butt" is a song by electronic music project Major Lazer from their second studio album Free the Universe. It features guest appearances from American recording artists Bruno Mars, Tyga and Jamaican dancer and rapper Mystic Davis. The song became the fourth single from the album after a remix featuring American rapper 2 Chainz was released on May 24, 2013. "Bubble Butt" is Major Lazer's second most commercially successful single in the United States to date, peaking at number 56 on the Billboard Hot 100.