The Collaborative International Dictionary of English v.0.48:

affiance \af*fi"ance\, v. t. [imp. & p. p. affianced
   ([a^]f*f[imac]"anst); p. pr. & vb. n. affiancing
   ([a^]f*f[imac]"an*s[i^]ng).] [Cf. OF. afiancier, fr.
   afiance.]
   1. To betroth; to pledge one's faith to for marriage, or
      solemnly promise (one's self or another) in marriage.
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            To me, sad maid, he was affianced.    --Spenser.
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   2. To assure by promise. [Obs.] --Pope.
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The Collaborative International Dictionary of English v.0.48:

Affiance \Af*fi"ance\ ([a^]f*f[imac]"ans), n. [OE. afiaunce
   trust, confidence, OF. afiance, fr. afier to trust, fr. LL.
   affidare to trust; ad + fidare to trust, fr. L. fides faith.
   See Faith, and cf. Affidavit, Affy, Confidence.]
   1. Plighted faith; marriage contract or promise. [archaic]
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   2. Trust; reliance; faith; confidence. [archaic]
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            Such feelings promptly yielded to his habitual
            affiance in the divine love.          --Sir J.
                                                  Stephen.
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            Lancelot, my Lancelot, thou in whom I have
            Most joy and most affiance.           --Tennyson.
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WordNet (r) 3.0 (2006):

affiance
    v 1: give to in marriage [syn: betroth, engage, affiance,
         plight]

Bouvier's Law Dictionary, Revised 6th Ed (1856):

AFFIANCE, contracts. From affidare or dare fidem, to give a pledge. A
plighting of troth between a man and woman. Litt. s. 39. Pothier, Traite du
Mariage, n. 24, defines it to be a an agreement by which a man and a woman
promise each other that they will marry together. This word is used by some
authors as synonymous with marriage. Co. Litt. 34, a, note 2. See Dig. 23, 1
Code 5, 1, 4; Extrav. 4, 1.