In French, a complement or something that completes; in English, an archaic or borrowed term meaning completion or accompaniment.
From French complètement, derived from Latin completum (neuter past participle of complere, 'to fill up'). Borrowed from French where it means 'completely' or 'complement.'
Completement shows how English borrowed heavily from French after 1066 when Norman invaders brought their language, and we still sprinkle French words through English even when we have perfectly good English equivalents!
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