CALM, an adjective meaning peaceful, quiet; particularly used of the weather, free from wind or storm, or of the sea, opposed to rough. The word appears in French calme, through which it came into English, in Spanish, Portuguese and Italian calma. Most authorities follow Diez (Etym. Wörterbuch der romanischen Sprachen) in tracing the origin to the Low Latin cauma, an adaptation of Greek , burning heat, , to burn. The Portuguese calma has this meaning as well as that of quiet. The connexion would be heat of the day, rest during that period, so quiet, rest, peacefulness. The insertion of the l, which in English pronunciation disappears, is probably due to the Latin calor, heat, with which the word was associated.
Note - this article incorporates content from Encyclopaedia Britannica, Eleventh Edition, (1910-1911)