Old English wæter, from Proto-Germanic *watr- (source also of Old Saxon watar, Old Frisian wetir, Dutch water, Old High German wazzar, German Wasser, Old Norse vatn, Gothic wato "water"), from PIE *wod-or, suffixed form of root *wed- (1) "water; wet."
To keep (one's) head above water in the figurative sense is recorded from 1742. Water cooler is recorded from 1846; water polo from 1884; water torture from 1928. Linguists believe PIE had two root words for water: *ap- and *wed-. The first (preserved in Sanskrit apah as well as Punjab and julep) was "animate," referring to water as a living force; the latter referred to it as an inanimate substance. The same probably was true of fire (n.).
late 14c., "a small private room for study or prayer," from Old French closet "small enclosure, private room," diminutive of clos "enclosure," from Latin clausum "closed space, enclosure, confinement," from neuter past participle of claudere "to shut" (see close (v.)).
In Matthew vi.6 it renders Latin cubiculum "bedchamber, bedroom," Greek tamieion "chamber, inner chamber, secret room." Modern sense of "small side-room for storage" is first recorded 1610s.
The adjective is from 1680s, "private, done in seclusion;" from 1782 as "fitted only for scholarly seclusion, not adopted to the conditions of practical life." The meaning "secret, not public, unknown" is recorded from 1952, first of alcoholism but by 1970s used principally of homosexuality; the phrase come out of the closet "admit something openly" is first recorded 1963, and lent a new meaning to the word out.