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

Fashion

Fashion, a general term for a currently popular style or practice, especially in clothing, foot wear, or accessories. Fashion references to anything that is the current trend in look and dress up of a person...

, usually emblazoned with punk

Punk subculture

The punk subculture includes a diverse array of ideologies, and forms of expression, including fashion, visual art, dance, literature, and film, which grew out of punk rock.-History:...

 iconography.

Flaps are usually constructed by hand with the most suitable material available, often canvas

Canvas

Canvas is an extremely heavy-duty plain-woven fabric used for making sails, tents, marquees, backpacks, and other items for which sturdiness is required. It is also popularly used by artists as a painting surface, typically stretched across a wooden frame...

 or denim

Denim

Denim is a rugged cotton twill textile, in which the weft passes under two or more warp threads. This produces the familiar diagonal ribbing identifiable on the reverse of the fabric, which distinguishes denim from cotton duck. Denim has been in American usage since the late 18th century...

 of discarded clothing. Water resistant and thermally insulating flaps are desirable for long distance rail travel

Freighthopping

Freighthopping or train hopping is the act of surreptitiously hitching a ride on a railroad freight car. In the United States, this became a common means of transportation following the American Civil War as the railroads began pushing westward, especially among migrant workers who became known as...

and camping. Flaps may be stitched to clothing, or suspended from the belt.