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.
