The Collaborative International Dictionary of English v.0.48:

Disconcert \Dis`con*cert"\, v. t. [imp. & p. p. Disconcerted;
   p. pr. & vb. n. Disconcerting.] [Pref. dis- + concert: cf.
   OF. desconcerter, F. d['e]concerter.]
   1. To break up the harmonious progress of; to throw into
      disorder or confusion; as, the emperor disconcerted the
      plans of his enemy.
      [1913 Webster]

   2. To confuse the faculties of; to disturb the composure of;
      to discompose; to abash.
      [1913 Webster]

            The embrace disconcerted the daughter-in-law
            somewhat, as the caresses of old gentlemen unshorn
            and perfumed with tobacco might well do.
                                                  --Thackeray.

   Syn: To discompose; derange; ruffle; confuse; disturb;
        defeat; frustrate.
        [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

Disconcert \Dis`con*cert"\, n.
   Want of concert; disagreement. --Sir W. Temple.
   [1913 Webster]

WordNet (r) 3.0 (2006):

disconcert
    v 1: cause to feel embarrassment; "The constant attention of the
         young man confused her" [syn: confuse, flurry,
         disconcert, put off]
    2: cause to lose one's composure [syn: upset, discompose,
       untune, disconcert, discomfit]