in Latin, the form of com- "together, with" in compounds with stems beginning in vowels, h-, and gn-; see com-. Taken in English from 17c. as a living prefix meaning "together, mutually, in common," and used promiscuously with native words (co-worker) and Latin-derived words not beginning with vowels (codependent), including some already having it (co-conspirator).
mid-15c., dominaunt, in ordre dominaunt, the name of the fourth order of angels, from Old French dominant (13c.) and directly from Latin dominantem (nominative dominans), present participle of dominari "to rule, dominate, to govern," from dominus "lord, master," from domus "house" (from PIE root *dem- "house, household").
From 1530s as "exercising rule or chief authority;" by 1854 as "having a controlling effect or influence." Music sense "based on or belonging to the fifth tone of the scale" is from 1819. Sexual bondage sense "exerting control over the submissive partner" is by c. 1960. The noun is first recorded 1819, earliest in the musical sense. Related: Dominantly.