sewer (n.1) Look up sewer at Dictionary.com
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-Roman *exaquaria (source of 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.
sewer (n.2) Look up sewer at Dictionary.com
"one who sews," late 14c., agent noun from sew (v.).