c.1374, from O.Fr.
tranquilite (12c.), from L.
tranquillitatem (nom.
tranquillitas) "tranquilness," from
tranquillus "tranquil," perhaps from
trans- "over" (here in sense of "exceedingly") + a root related to
quies "rest" (see
quiet). The adj.
tranquil is attested from 1604, from Fr.
tranquille (1470).
Tranquilize formed in Eng. 1623;
tranquilizer "sedative" is from 1824 (first reference is to ground ivy); in reference to one of a large group of anti-anxiety drugs, it is first recorded 1956.