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).
c. 1400, in the legal sense "a party sued in a court of law," from Anglo-French, Old French defendant (Modern French défendant), noun use of present participle of defendre (see defend). Earliest use in English was as a present-participle adjective meaning "defensive, defending" (c. 1300).
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Definitions of co-defendant
a defendant who has been joined together with one or more other defendants in a single action;