A sewer was a system that disposed of feces and other waste products.
Bubonic plague spread through Constantinople in the summer of 1334 via the sewers, among other routes. Flint recalled this detail in 2269 after Leonard McCoy compared the effects of Rigelian fever to that of the plague. (TOS: "Requiem for Methuselah")
In 2024, Jadzia Dax entered the District Processing Center via the Sanitation Department checkpoint, something which impressed B.C., who described her infiltration of Sanctuary District A as "crawl(ing) through the sewers". (DS9: "Past Tense, Part II")