Английский Этимологический словарь - tranquility

- c.1380, from O.Fr. tranquilitй, 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. 1623; tranquilizer "sedative" is from 1824; in reference to one of a large group of anti-anxiety drugs, it is first recorded 1956.