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).
late 14c., from Old French eternel "eternal," or directly from Late Latin aeternalis, from Latin aeternus "of an age, lasting, enduring, permanent, everlasting, endless," contraction of aeviternus "of great age," from aevum "age" (from PIE root *aiw- "vital force, life; long life, eternity").
Used since Middle English both of things or conditions without beginning or end and things with a beginning only but no end. A parallel form, Middle English eterne, is from Old French eterne (cognate with Spanish eterno), directly from Latin aeternus. Related: Eternally. The Eternal (n.) for "God" is attested from 1580s.
<a href="https://www.etymonline.com/word/coeternal">Etymology of coeternal by etymonline</a>
Harper, D. (n.d.). Etymology of coeternal. Online Etymology Dictionary. Retrieved $(datetime), from https://www.etymonline.com/word/coeternal