affiance (v.) Look up affiance at Dictionary.com
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 "faithful," from PIE root *bheidh- "to trust" (see faith).

From mid-16c. especially "to promise in marriage." The earlier form of the word was affy (Middle English affien "to trust, have faith; have faith in" c. 1300), from Old French afier. Related: Affianced; affiancing.