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1.
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.
'Man that shitpipe was mad sewerfresh, terds the whole way.'
by Sayhelloagain April 12, 2010