What is the origin of serein?

Serein “fine rain falling after sunset from a cloudless sky” is a borrowing from French, from Middle French serain “evening, nightfall.” If you listened to our recent Word of the Day podcast about serotinal “occurring in late summer,” the ultimate origin of serein should look familiar: the term derives from Latin sērum “a late hour,” from the adjective sērus “late.” A common misconception is that serein is related to serene “peaceful, clear, unclouded,” but while the words are similar in appearance and definition, they are likely unconnected. Serene instead derives from Latin serēnus “cheerful, tranquil; (of weather) clear, unclouded,” and a derivative noun, serēnum, means “fine weather.” Serein was first recorded in English in the late 1860s.