Bum flap
The bum or butt flap is a piece of removable material that hangs from the waist to cover the rear end. The flap is of ambiguous origin, but probably was originally intended to reduce abrasive wear to pants. Pavement can rapidly wear the seat of pants, and flaps act as replaceable buffer material. They also provide some protection from damp or cold surfaces. The freely swinging flap dries faster than wet pants, and can be easier to clean. Bum flaps are often worn as a superfluous fashion statement, usually emblazoned with punk iconography.
Flaps are usually constructed by hand with the most suitable material available, often canvas or denim of discarded clothing. Water resistant and thermally insulating flaps are desirable for long distance rail travel and camping. Flaps may be stitched to clothing, or suspended from the belt.