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:

disconcerting \disconcerting\ adj.
   hard to deal with; causing uncertainty or confusion about how
   to act or react.

   Syn: awkward, embarrassing, off-putting, sticky, tight,
        unenviable.
        [WordNet 1.5]

WordNet (r) 3.0 (2006):

disconcerting
    adj 1: causing an emotional disturbance; "his disconcerting
           habit of greeting friends ferociously and strangers
           charmingly"- Herb Caen; "an upsetting experience" [syn:
           disconcerting, upsetting]