serenity (n.)

mid-15c., of a day, "clear, fair, calm," from Old French serein and directly from Latin serenus "peaceful, calm, clear, unclouded" (of weather); figuratively "cheerful, glad, tranquil"(from PIE root *ksero- "dry," source also of Greek xeros "dry, arid;" see xerasia).

In English, the word has been applied to persons, characters, etc. since 1630s: "tranquil, unruffled." Related: Serenely. Middle English also had serenous (mid-15c.), of places, "having clear, fair weather."