late 14c., 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). Tranquilize formed in Eng. 1620s; tranquilizer