Source : Webster's Revised Unabridged Dictionary (1913)

Unpleasant \Un*pleas"ant\, a.
   Not pleasant; not amiable or agreeable; displeasing;
   offensive. -- {Un*pleas"ant*ly}, adv. -- {Un*pleas"ant*ness},
   n.

Source : WordNet®

unpleasantness
     n 1: the feeling caused by disagreeable stimuli; one pole of a
          continuum of states of feeling [ant: {pleasantness}]
     2: the quality of giving displeasure; "the recent
        unpleasantness of the weather" [ant: {pleasantness}]