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]