Source : Webster's Revised Unabridged Dictionary (1913)

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.

   2. To confuse the faculties of; to disturb the composure of;
      to discompose; to abash.

            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.

Source : WordNet®

disconcerting
     adj : causing an emotional disturbance; "his disconcerting habit
           of greeting friends ferociously and strangers
           charmingly"- Herb Caen; "an upsetting experience" [syn:
            {upsetting}]