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]

Moby Thesaurus II by Grady Ward, 1.0:

101 Moby Thesaurus words for "disconcert":
   abash, addle, addle the wits, agitate, appall, astound, baffle,
   balk, ball up, becloud, bedazzle, befuddle, bewilder, blast,
   bother, brave, bug, cast down, chagrin, challenge, checkmate,
   circumvent, cloud, confound, confront, confuse, contravene,
   counter, counteract, countermand, counterwork, cross, dash, daze,
   dazzle, defeat, defy, destroy, discombobulate, discomfit,
   discompose, discountenance, dish, dismay, disorganize, disorient,
   disquiet, disrupt, disturb, electrify, elude, embarrass, entangle,
   faze, flummox, flurry, fluster, flutter, fog, foil, frustrate,
   fuddle, fuss, humiliate, jar, jolt, knock the chocks, maze, mist,
   mix up, moider, mortify, muddle, nonplus, perplex, perturb, pother,
   put out, puzzle, raise hell, rattle, rock, ruffle, ruin, sabotage,
   scotch, shake, shake up, shock, spike, spoil, stagger, stir,
   stonewall, stump, take aback, throw into confusion, thwart,
   trouble, unsettle, upset