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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