serenity (n.)youdaoicibaDictYouDict[serenity 词源字典]
1530s, of weather, 1590s, of persons, from Middle French sérénité, from Latin serenitatem (nominative serenitas) "clearness, serenity," from serenus (see serene). Earliest use (mid-15c.) was as a title of honor for kings, probably from the similar use of Latin serenitas, applied to Roman emperors, later popes.[serenity etymology, serenity origin, 英语词源]