The Collaborative International Dictionary of English v.0.48:

Ill-natured \Ill`-na"tured\, a.
   1. Of habitual bad temper; having an unpleasant disposition;
      surly; disagreeable; cross; peevish; fractious; crabbed;
      -- of people; as, an ill-natured person; an ill-natured
      disagreeable old man. Opposite of good-natured.
      [Narrower terms: argumentative, contentious,
      disputatious, disputative, litigious : {atrabilious,
      bilious, dyspeptic, liverish : {bristly, prickly,
      snappish, splenetic, waspish : {cantankerous, crotchety,
      ornery : {choleric, irascible, hotheaded, hot-headed,
      hot-tempered, quick-tempered, short-tempered : {crabbed,
      crabby, cross, fussy, fussbudgety, grouchy, grumpy,
      bad-tempered, ill-tempered: cranky, fractious,
      irritable, peevish, peckish, pettish, petulant, testy,
      tetchy, techy : {crusty, curmudgeonly, gruff, ill-humored,
      ill-humoured: dour, glowering, glum, moody, morose,
      saturnine, sour, sullen : {feisty, touchy : {huffish,
      sulky: misanthropic, misanthropical : {misogynous :
      {shirty, snorty ill-tempered or annoyed): shrewish,
      nagging, vixenish : {surly, ugly ] Also See: {unpleasant.
      [1913 Webster + WordNet 1.5]

   2. Dictated by, or indicating, ill nature; spiteful. "The
      ill-natured task refuse." --Addison.
      [1913 Webster]

   3. Intractable; not yielding to culture. [R.] "Ill-natured
      land." --J. Philips.

   3. not to one's liking; unpleasant; disagreeable. Opposite of
      agreeable. [WordNet sense 2] [Narrower terms: annoying,
      galling, chafing, irritating, nettlesome, pesky,
      pestiferous, pestilent, plaguy, plaguey, teasing,
      vexatious, vexing; nerve-racking, nerve-wracking,
      stressful, trying ]

   Syn: disagreeable.
        [WordNet 1.5] -- {Ill`-na"tured*ly, adv. --
        Ill`-na"tured*ness, n.
        [1913 Webster]

WordNet (r) 3.0 (2006):

ill-natured
    adj 1: having an irritable and unpleasant disposition [ant:
           good-natured]

Moby Thesaurus II by Grady Ward, 1.0:

21 Moby Thesaurus words for "ill-natured":
   bad-tempered, bitter, crabbed, cranky, cross, crusty, dour,
   evil-disposed, evil-tempered, gloomy, ill-affected,
   ill-conditioned, ill-disposed, ill-humored, ill-intentioned,
   ill-tempered, malevolent, morose, petulant, sour, sulky