sewerage (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).

"the refuse matter which passes through sewers," 1818, probably from the apparent base of sewer (n.1) + -age. There was a verb sew "to drain, to draw off water," attested from late 15c., but by 19c. it seems to have survived only in provincial dialect, and OED writes, "It was prob. framed without any knowledge of the verb as having been actually used." Compare sewerage.