c.1400, "conduit," from Anglo-French sewere, Old North French sewiere "sluice from a pond" (13c.), literally "something that makes water flow," from shortened form of Gallo-Romance *exaquaria (cf. Middle French esseveur), from Latin ex- "out" (see ex-) + aquaria, fem. of aquarius "pertaining to water," from aqua "water" (see aqua-).
Specifically of underground channels for wastewater from c.1600; figurative use of this is from 1640s.
"one who sews," late 14c., agent noun from sew (v.).
A vein or artery: if I had put it right in the sewer instead of skin popping (1960s+ Narcotics)