"A fat bum" would do in English, or "Fat buttocks" but not "A fat buttocks".
I think they are being polite. “A fat @rse”!
"Buttocks" is plural though, so it definitely doesn't work with the singular "A". Maybe just leave it at "Fat buttocks"?
Buttocks also feels very much like American English versus British English which would use bum or backside.
as posted in a different thread, North Americans, including us Canadians, would say "butt" before we would say buttocks.
Yeah, the grammar definitely doesn't feel right here. Not sure if it's supposed to sound like a veiled way of slipping in 'fat @rse' or just 'A fat bum'.