Opposite of BBW or big beautiful woman.
Not all plumpers are beautiful. Some are....some ain't.
Dude2: 'Sally?!!! Are you kidding...she's a total BUW!"
Dude1: 'Heh, don't you mean BBW?'
Dude2: 'No, I mean what I said...She's a 'big ugly woman'...a 'B.U.W.'
Dude1: 'Haha, I've not heard that one before!'
Dude2: 'I'm not surprised....I just made it up!'