Bum fluff is literally sparse and whispy facial hairs on an adolescent boy, (Or Asian girl)... Tee hee! Which resemble peach fuzz, under-bed lint, gossimar threads... or any fine downy substance.
"You look a right cunt with that bum fluff on thy face mate! Grow up and stop trying to look like a blonde girl's arse!!!!" * Mr Kipling's Diaries.*