The Collaborative International Dictionary of English v.0.48:

Obnoxious \Ob*nox"ious\ ([o^]b*n[o^]k"sh[u^]s), a. [L. obnoxius;
   ob (see Ob-) + noxius hurtful. See Noxious.]
   1. Subject; liable; exposed; answerable; amenable; -- with
      to.
      [1913 Webster]

            The writings of lawyers, which are tied obnoxious to
            their particular laws.                --Bacon.
      [1913 Webster]

            Esteeming it more honorable to live on the public
            than to be obnoxious to any private purse. --Milton.
      [1913 Webster]

            Obnoxious, first or last,
            To basest things                      --Milton.
      [1913 Webster]

   2. Liable to censure; exposed to punishment; reprehensible;
      blameworthy. "The contrived and interested schemes of . .
      . obnoxious authors." --Bp. Fell.
      [1913 Webster]

            All are obnoxious, and this faulty land,
            Like fainting Hester, does before you stand
            Watching your scepter.                --Waller.
      [1913 Webster]

   3. Very offensive; odious; hateful; as, an obnoxious
      statesman; a minister obnoxious to the Whigs. --Burke.
      [1913 Webster] -- Ob*nox"ious*ly, adv. --
      Ob*nox"ious*ness, n. --South.
      [1913 Webster]

WordNet (r) 3.0 (2006):

obnoxious
    adj 1: causing disapproval or protest; "a vulgar and
           objectionable person" [syn: objectionable, obnoxious]

Moby Thesaurus II by Grady Ward, 1.0:

123 Moby Thesaurus words for "obnoxious":
   abhorrent, abject, abominable, arrant, atrocious, awful, base,
   beastly, beggarly, below contempt, beneath contempt, blameworthy,
   brutal, cheesy, contemptible, crude, crummy, debased, degraded,
   deplorable, depraved, despicable, detestable, dire, dirty,
   disgusting, dreadful, egregious, enormous, execrable, exposed,
   fetid, filthy, flagrant, forbidding, foul, fulsome, grave,
   grievous, gross, hateful, heinous, horrible, horrid, ignoble,
   infamous, invidious, lamentable, little, loathsome, lousy, low,
   low-down, lumpen, malodorous, mangy, mean, measly, mephitic,
   miasmal, miasmic, miserable, monstrous, nasty, nauseating,
   nefarious, noisome, notorious, noxious, objectionable, obscene,
   odious, offensive, open, outrageous, paltry, petty, pitiable,
   pitiful, poky, poor, prone, rank, rebarbative, regrettable,
   repellent, reprehensible, reptilian, repugnant, repulsive,
   revolting, revulsive, rotten, sad, scabby, scandalous, schlock,
   scrubby, scruffy, scummy, scurvy, sensitive, shabby, shameful,
   shocking, shoddy, sickening, small, sordid, squalid, stinking,
   subject, susceptible, terrible, too bad, unclean, unmentionable,
   vile, villainous, woeful, worst, worthless, wretched