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-14c., "that is such by its essence," from Late Latin essentialis, from essentia "being, essence," abstract noun formed (to translate Greek ousia "being, essence") from essent-, present participle stem of esse "to be," from PIE root *es- "to be." Meaning "pertaining to essence" is from late 14c., that of "constituting the essence of something" is from 1540s; that of "necessary" is from 1520s. Essentials "indispensable elements" is from early 16c. Related: Essentially.
<a href="https://www.etymonline.com/word/coessential">Etymology of coessential by etymonline</a>
Harper, D. (n.d.). Etymology of coessential. Online Etymology Dictionary. Retrieved $(datetime), from https://www.etymonline.com/word/coessential