Amongst those who explore sewers, its a way of defining a wastewater tunnels contents (sewage versus stormwater) while making yourself sound hardcore and cool, as opposed to sounding like someone who wades thru shit filled tunnels as a hobby.

Originally derived from the way cooled shit mist wafted out of an open sewer outfall on the Mississippi River and relieved explorers standing there from the heat of a Minnesotan summer.

A swimmer