also jar-head, "U.S. Marine," by 1985 (but in a biographical book with a World War II setting), from jar + head (n.). Also used as a general term of insult (by 1979) and by 1922 as a Georgia dialectal word for "mule."

"a standby, a reserve," 1952; see back up (v.). Specific reference to computing is from 1965.

"soldier who belongs to the reserves," 1872, from French réserviste, from reserver (see reserve (v.)).

of bottle or jar caps, 1959, from the verbal phrase; see twist (v.) + off (adv.).

"having a sharp, unpleasant effect," 1550s, present-participle adjective from jar (v.). Related: Jarringly.

liquid measure (in modern use commonly a quarter of a pint), late 13c., from Old French gille, a wine measure, and from Medieval Latin gillo "earthenware jar," words of uncertain origin, perhaps related to the source of gallon.

type of bell-jar, 1882, from French cloche "bell, bell glass" (12c.), from Late Latin clocca "bell" (see clock (n.1)). As a type of women's hat, recorded from 1907, so called from its shape.

group of islands off the north coast of Scotland, from Old Norse Orkney-jar "Seal Islands," from orkn "seal," which is probably imitative of its bark. With Old Norse ey "island" (compare Jersey). Related: Orcadian; Orkneyman.

1530s, "not restricted or withheld," from un- (1) "not" + past participle of reserve (v.). From 1713 as "open, frank." Related: Unreservedly.

earthenware mug, 1855, from German Stein, shortened form of Steinkrug "stone jug," from Stein "stone" (see stone (n.)) + Krug "jug, jar." Compare Old English stæne "pitcher, jug."