late 14c., "having no deficiency, wanting no part or element; perfect in kind or quality; finished, ended, concluded," from Old French complet "full," or directly from Latin completus, past participle of complere "to fill up, complete the number of (a legion, etc.)," transferred to "fulfill, finish (a task)," from com-, here probably as an intensive prefix (see com-), + plere "to fill" (from PIE root *pele- (1) "to fill").
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<a href="https://www.etymonline.com/word/compleat">Etymology of compleat by etymonline</a>
Harper, D. (n.d.). Etymology of compleat. Online Etymology Dictionary. Retrieved $(datetime), from https://www.etymonline.com/word/compleat