From Middle English fyancen, fyauncen, from Middle French fiancer.
Meanings
(obsolete) To betroth; to affiance.
he should become so lusty over a lady of such questionable motives? He was fianced, after all. Perhaps that was it. Since his engagement, he had abstained from any liaisons, feeling it was only proper in a man soon to be married 1993, Cindy Holbrook, A Daring Deception, page 91:
he[…]therfore fianced he his daughter 1569, Thomas Stocker, A righte noble and pleasant history of the successors of Alexander surnamed the Great: