Not to be confused with Britian's Special Air Service, S.A.S. refers to "sweaty ass syndrome", or ass-crack sweat.

Sweat from the lower back accumulates in the crack of one's ass (usually men) and forms wet stains on thier boxers, sometimes permanently if the sweat is mixed with fecal matter.

Usually treated with a "manpon", or wadded-up handful of toilet paper inserted into the asscrack.