Serene

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late Middle English (describing the weather or sky as ‘clear, fine, and calm’): from Latin serenus .

wiktionary

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From Middle English, borrowed from Latin serēnus(“clear, cloudless, untroubled”).

Old French serein(“evening”), Vulgar Latin *serānum (from substantive use of sērum, neuter of sērus(“late”)) + -ānus suffix.

etymonline

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serene (adj.)

mid-15c., "clear, calm," from Latin serenus "peaceful, calm, clear, unclouded" (of weather), figuratively "cheerful, glad, tranquil," from Proto-Italic *(k)sero- "dry," from PIE root *ksero- "dry," source also of Greek xeros "dry, arid" (see xerasia). In English, applied to persons since 1630s. Related: Serenely.