girl 1: woah, that bum flap goes great with his bondage pants  girl 2:
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
"girl 1: woah, that bum flap goes great with his bondage pants  girl 2:" What is the definition of "bum flap"?