co-

co- in Latin, the form of com- in compounds with stems beginning in vowels and h- and gn- (see COM- (Cf. com-)). Taken in English from 17c. as a living prefix meaning “together, mutually, in common,” and used promiscuously with native words and Latin-derived words not beginning with vowels, sometimes even with words already having it (e.g. co-conspiritor).