Often mistaken with anal fluff (different thing etirely)... After a period of certain unhygienically living, a small amount of dust, cat hairs (dog hairs. human hair.. whatever) and sweat congeals in the belly button area of the body and stays there until one either washes (thus terminating the period of unhygienically living and smell) or the lucky other half (or embarassed and soon to fail one night stand) picks it out and unvariably smells it.... it smells of bum.
Phwoar! your bum fluf smells of ass!
What's fluff? Oh! It smells of bottoms. Must be bum fluff again.