The Collaborative International Dictionary of English v.0.48:

Unpleasant \Un*pleas"ant\, a.
   Not pleasant; not amiable or agreeable; displeasing;
   offensive. -- Un*pleas"ant*ly, adv. -- Un*pleas"ant*ness,
   n.
   [1913 Webster]

WordNet (r) 3.0 (2006):

unpleasant
    adj 1: disagreeable to the senses, to the mind, or feelings ;
           "an unpleasant personality"; "unpleasant repercussions";
           "unpleasant odors" [ant: pleasant]

Moby Thesaurus II by Grady Ward, 1.0:

36 Moby Thesaurus words for "unpleasant":
   abhorrent, bad, bitter, disagreeable, dislikable, displeasing,
   distasteful, foul-tasting, ill-flavored, intolerable, mislikable,
   odious, rotten, sour, thankless, ugly, unalluring, unappealing,
   unappetizing, unattractive, uncongenial, undelectable, undelicious,
   undesirable, unengaging, unenjoyable, unhappy, uninviting,
   unlikable, unlovable, unpalatable, unpleasing, unsavory,
   untasteful, untasty, unwelcome