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 15c., "contiguous, bordering; close, nearby," from Latin adiacentem (nominative adiacens) "lying at," present participle of adiacere "lie at, border upon, lie near," from ad "to" (see ad-) + iacēre "to lie, rest," related to iacere "to throw; to lay" (from PIE root *ye- "to throw, impel"). Only of things, never of persons or animals. Adjacent, properly, is near but not necessarily in contact; adjoining is so as to touch. Latin adiacentia meant "the neighborhood."
<a href="https://www.etymonline.com/word/coadjacent">Etymology of coadjacent by etymonline</a>
Harper, D. (n.d.). Etymology of coadjacent. Online Etymology Dictionary. Retrieved $(datetime), from https://www.etymonline.com/word/coadjacent