It is a piece of fabric about 13x13 hanging from the waist covering the butt (the front/sides)sometimes with d-rings, usually plaid or pinstripe, mostly worn by the punk community.

A person with an outwardly dominant, masculine, or aggressive personality, but who also enthusiastically takes on the submissive or "bottom" role in the bedroom.

When an arse is so emmense that the cheeks turn into flaps. Can be used as a negative term.

The flaps inside your anus.

It's underwear. Is there anything else to know?

a bum flap is wen a girl has a really really short skirt, thats rolled over so many times, its like a really thick belt, and so instead of being a skirt .. is a bumflap! (you see the link...), i.e. nothings left to the imagination, especially on a windy day...bumflaps are very common in secondary schools with the *were not even gonna hide were desperate for a shag* kinda girls