The Collaborative International Dictionary of English v.0.48:

Betroth \Be*troth"\, v. t. [imp. & p. p. Betrothed; p. pr. &
   vb. n. Betrothing.] [Pref. be- + troth, i. e., truth. See
   Truth.]
   1. To contract to any one for a marriage; to engage or
      promise in order to marriage; to affiance; -- used esp. of
      a woman.
      [1913 Webster]

            He, in the first flower of my freshest age,
            Betrothed me unto the only heir.      --Spenser.
      [1913 Webster]

            Ay, and we are betrothed.             --Shak.
      [1913 Webster]

   2. To promise to take (as a future spouse); to plight one's
      troth to.
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            What man is there that hath betrothed a wife, and
            hath not taken her?                   --Deut. xx. 7.
      [1913 Webster]

   3. To nominate to a bishopric, in order to consecration.
      --Ayliffe.
      [1913 Webster]

WordNet (r) 3.0 (2006):

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

Easton's 1897 Bible Dictionary:

Betroth
   to promise "by one's truth." Men and women were betrothed when
   they were engaged to be married. This usually took place a year
   or more before marriage. From the time of betrothal the woman
   was regarded as the lawful wife of the man to whom she was
   betrothed (Deut. 28:30; Judg. 14:2, 8; Matt. 1:18-21). The term
   is figuratively employed of the spiritual connection between God
   and his people (Hos. 2:19, 20).