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1520s, "to promise," from Old French afiancier "to pledge, promise, give one's word," from afiance (n.) "confidence, trust," from afier "to trust," from Late Latin affidare, from ad- "to" (see ad-) + fidare "to trust," from fidus (see affidavit). From mid-16c. especially "to promise in marriage." Related: Affianced; affiancing.