The Collaborative International Dictionary of English v.0.48:

Crestfallen \Crest"fall`en\ (-f?l`'n), a.
   1. With hanging head; hence, dispirited; dejected; cowed.
      [1913 Webster]

            Let it make thee crestfullen;
            Ay, and allay this thy abortive pride. --Shak.
      [1913 Webster]

   2. Having the crest, or upper part of the neck, hanging to
      one side; -- said of a horse.
      [1913 Webster]

WordNet (r) 3.0 (2006):

crestfallen
    adj 1: brought low in spirit; "left us fatigued and deflated
           spiritually" [syn: chapfallen, chopfallen,
           crestfallen, deflated]

Moby Thesaurus II by Grady Ward, 1.0:

53 Moby Thesaurus words for "crestfallen":
   abashed, ashamed, baffled, balked, betrayed, bilked, blasted,
   blighted, blushing, brooding, broody, chagrined, chapfallen,
   crossed, crushed, dashed, defeated, disappointed, dished,
   disillusioned, dissatisfied, dumpish, embarrassed, foiled,
   frustrated, glum, grum, hangdog, humbled, humiliated, ill done-by,
   ill-served, let down, long-faced, moodish, moody, mopey, moping,
   mopish, morose, mortified, mumpish, out of countenance, red-faced,
   regretful, shamed, shamefaced, shamefast, sorely disappointed,
   soured, sulky, sullen, thwarted