Why is the ninth month called September?
mid-12c., "large group of people," from Old French compagnie "society, friendship, intimacy; body of soldiers" (12c.), from Late Latin companio (see companion). Meaning "companionship" is from late 13c. Sense of "business association" first recorded 1550s, having earlier been used in reference to trade guilds (c.1300). Meaning "subdivision of an infantry regiment" is from 1580s. Abbreviation co. dates from 1670s.