Originating from leprechaun rectums in south ireland, bum fluff is the fibres of sweaty underwear that sticks to the scrotum and "Batty criece" when a respectable amount of sweat has been produced that day. On a hot day, people may be seen pulling their underwear out of the arse with their hands, this usually occurs when bum fluff is present.
"Ooh it's hot today isn't it?" *pulls underwear from bum crack*
"I think i better have a shower, my bum is currently covered in smelly wool."