A piece of cloth attached to the back of your pants or jacket covering the lower portion of your back. Usually screen printed with punk rock logos, insignias, and covered with conical or pyramid studs. Worn by punk rockers of all kinds. Extra bonus points are rewarded if your butt-flap attaches to a studded vest with pyramid or conical studs. The first known sighting of a butt-flap was as ABCNORIO in the lowe east side of New York city in 1989, although it is suspected to have originated in London at an earlier date.
"Dude, that bass player from Impaled has a killer butt-flap! Check out his vest"