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).
early 14c. "to send to the bottom" (transitive); late 14c., "to sink or fall" (intransitive), from Old French fondrer "collapse; submerge, sink, fall to the bottom" (Modern French fondrier), from fond "bottom" (12c.), from Latin fundus "bottom, foundation" (see fund (n.)). Not especially of ships in Middle English, where it typically meant "fall to the ground." Figurative use from 1580s. Related: Foundered; foundering.