area of darker skin surround a womans anal sphincter muscle.
originated from the thought that it is there because said female has not wiped her arse sufficiently over a long period of time, and the turd has rubbed into the skin. This thought was obviously incorrect. Term first used 1990, Blackpool

Apparently, can be eliminated by procedure called anal bleaching

When you eat dirt for the first time and leave your family to build a shack in the woods