Why is the ninth month called September?
"calm, tranquil," c.1600, past participle adjective frome compose (v.). Related: Composedly; composedness.
c.1400, compousen, from Old French composer "put together, arrange, write" a work (12c.), from com- "with" (see com-) + poser "to place," from Late Latin pausare "to cease, lay down," ultimately from Latin ponere "to put, place" (see position (n.)). Meaning influenced in Old French by componere (see composite). Musical sense is from 1590s. Related: Composed; composing.