A piece of cloth (usually a dark color, most likely black) with a logo or picture of a band on it that hangs over a "punk" persons ass, therefore supposedly showing their undying gratitude for the band. Can also be worn over frontal region, in which it is called a "cock flap", even on females.
1]Dude, look at that wannabe punk rocker! He's wearing an ass flap!
2]Yeah, and a cock flap too!
1]What a tard.