Serenity

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late Middle English: from Old French serenite, from Latin serenitas, from serenus ‘clear, fair’ (see serene).

wiktionary

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From Middle English serenyte, from Old French serenité, from Latin serēnitās, equivalent to serene +‎ -ity.

etymonline

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serenity (n.)

1530s, of weather, 1590s, of persons, from 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.