seat
n.
1
"thing to sit on; act of sitting," c.1200, from Old Norse sæti "seat, position," from Proto-Germanic *sæt- (cognates: Old High German saze, Middle Dutch gesaete "seat," Old High German gisazi, German Gesäß "buttocks"), from PIE root *sed- (1) "to sit" (see sit). Meaning "posterior of the body" (the sitting part) is from c.1600; sense of "part of a garment which covers the buttocks" is from 1835. Seat belt is from 1915, originally in airplanes.
2
"residence, abode, established place," late 13c., extended use of seat, n.1, influenced by Old French siege "seat, established place," and Latin sedes "seat." Meaning "city in which a government sits" is attested from c.1400. Sense of "right of taking a place in a parliament or other legislative body" is attested from 1774. Old English had sæt "place where one sits in ambush," which also meant "residents, inhabitants," and is the source of the -set in Dorset and Somerset.
v.
1570s, "to be in a certain position" (implied in seated), from seat, n.2. Of diseases, in the body, from 1610s (hence deep-seated). Meaning "to cause to sit in a seat" is from 1610s, from seat (n.1). Related: Seated; seating.