sewage n.

c. 1400, seuer, "conduit, trench, or ditch used for drainage" (of surface water or marshland), from Anglo-French sewere (early 14c.), Old North French sewiere "sluice from a pond" (13c.), literally "something that makes water flow." From late 13c. in surnames (Robertus Atte Suor). Also compare Anglo-Latin sewera, suera. These are from a shortened form of Gallo-Roman *exaquaria (source of Old French esseveur), from Latin ex "out" (see ex-) + aquaria, fem. of aquarius "pertaining to water," from aqua "water" (from PIE root *akwa- "water"). For form evolution, compare ewer, from Latin aquarius.

After c. 1600 the sense of "underground channel for wastewater" emerged and predomination, especially "a public drain; a conduit or canal constructed to carry off waste water, etc." Figurative use of this is from 1640s. Sewer rat, the common brown rat when infesting sewers, is from 1861.

word-forming element in nouns of act, process, function, condition, from Old French and French -age, from Late Latin -aticum "belonging to, related to," originally neuter adjectival suffix, from PIE *-at- (source of Latin -atus, past participle suffix of verbs of the first conjugation) + *-(i)ko-, secondary suffix forming adjectives (see -ic).

"drainage sewers collectively; the process or system of collecting refuse and removing it by means of sewers;" 1832; see sewer (n.1) + -age. Compare sewage.