The Collaborative International Dictionary of English v.0.48:

Disappoint \Dis`ap*point"\, v. t. [imp. & p. p. Disapointed;
   p. pr. & vb. n. Disappointing.] [OF. desapointier, F.
   d['e]sappointer; pref. des- (L. dis-) + apointier, F.
   appointier, to appoint. See Appoint.]
   1. To defeat of expectation or hope; to hinder from the
      attainment of that which was expected, hoped, or desired;
      to balk; as, a man is disappointed of his hopes or
      expectations, or his hopes, desires, intentions,
      expectations, or plans are disappointed; a bad season
      disappoints the farmer of his crops; a defeat disappoints
      an enemy of his spoil.
      [1913 Webster]

            I was disappointed, but very agreeably. --Macaulay.
      [1913 Webster]

   Note: Disappointed of a thing not obtained; disappointed in a
         thing obtained.
         [1913 Webster]

   2. To frustrate; to fail; to hinder of result.
      [1913 Webster]

            His retiring foe
            Shrinks from the wound, and disappoints the blow.
                                                  --Addison.

   Syn: To tantalize; fail; frustrate; balk; baffle; delude;
        foil; defeat. See Tantalize.
        [1913 Webster]

WordNet (r) 3.0 (2006):

disappoint
    v 1: fail to meet the hopes or expectations of; "Her boyfriend
         let her down when he did not propose marriage" [syn:
         disappoint, let down]

Moby Thesaurus II by Grady Ward, 1.0:

54 Moby Thesaurus words for "disappoint":
   awaken, baffle, balk, beat, bilk, break the spell,
   burst the bubble, cast down, circumvent, correct, cross, dash,
   debunk, deceive, defeat, defeat expectation, disabuse, discontent,
   disenchant, disgruntle, dishearten, disillude, disillusion,
   disillusionize, displease, dissatisfy, drive to despair, enlighten,
   expose, fail, foil, frustrate, let down, let down easy, let in on,
   mislead, prick the bubble, put out, put straight, ruin, set right,
   set straight, show up, stand up, tantalize, tease, tell the truth,
   thwart, unblindfold, uncharm, undeceive, undo, unspell, wake up