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"
Those goofy squares of cloth, sometimes cut from an old tshirt or some other random piece of cloth (but now available at hot topic probably) that alot of street punks wear dangling from the back of their pants.
Freddie had to lift his buttflap out of the way to get his H out of his back pocket.