The Collaborative International Dictionary of English v.0.48:

Dishearten \Dis*heart"en\ (d[i^]s*h[aum]rt"'n), v. t. [imp. & p.
   p. Disheartened (d[i^]s*h[aum]rt"'nd); p. pr. & vb. n.
   Disheartening.] [Pref. dis- + hearten.]
   To discourage; to deprive of courage and hope; to depress the
   spirits of; to deject.
   [1913 Webster]

         Regiments . . . utterly disorganized and disheartened.
                                                  --Macaulay.

   Syn: To dispirit; discourage; depress; deject; deter;
        terrify.
        [1913 Webster]

WordNet (r) 3.0 (2006):

dishearten
    v 1: take away the enthusiasm of [syn: dishearten, put off]
         [ant: cheer, embolden, hearten, recreate]

Moby Thesaurus II by Grady Ward, 1.0:

61 Moby Thesaurus words for "dishearten":
   anticipate, avert, awe, bar, beat down, cast down, chill, damp,
   dampen, dampen the spirits, darken, dash, daunt, debar, deflect,
   deject, demoralize, depress, deter, disappoint, discontent,
   discourage, disgruntle, disparage, dispirit, displease, dissatisfy,
   estop, exclude, faze, fend, fend off, forbid, foreclose, forestall,
   help, keep from, keep off, knock down, lower, lower the spirits,
   obviate, oppress, overawe, preclude, press down, prevent, prohibit,
   put out, repel, rule out, sadden, save, shake, sink, stave off,
   stop, turn aside, ward off, weigh heavy upon, weigh upon