The Collaborative International Dictionary of English v.0.48:

Noxious \Nox"ious\, a. [L. noxius, fr. noxa harm; akin to nocere
   to harm, hurt. Cf. Nuisance, Necromancy.]
   [1913 Webster]
   1. Hurtful; harmful; baneful; pernicious; injurious;
      destructive; unwholesome; insalubrious; as, noxious air,
      food, or climate; pernicious; corrupting to morals; as,
      noxious practices or examples.
      [1913 Webster]

            Too frequent an appearance in places of public
            resort is noxious to spiritual promotions. --Swift.
      [1913 Webster]

   2. Guilty; criminal. [R.]
      [1913 Webster]

            Those who are noxious in the eye of the law. --Abp.
                                                  Bramhall.
      [1913 Webster]

   Syn: Noisome; hurtful; harmful; injurious; destructive;
        pernicious; mischievous; corrupting; baneful;
        unwholesome; insalubrious. See Noisome.
        [1913 Webster] -- Nox"ious*ly, adv. --
        Nox"ious*ness, n.
        [1913 Webster]

WordNet (r) 3.0 (2006):

noxious
    adj 1: injurious to physical or mental health; "noxious chemical
           wastes"; "noxious ideas" [ant: innocuous]

Moby Thesaurus II by Grady Ward, 1.0:

162 Moby Thesaurus words for "noxious":
   abhorrent, abominable, bad, bad for, bad-smelling, baleful,
   baneful, barfy, base, beastly, below contempt, beneath contempt,
   bitchy, brackish, catching, cloying, communicable, contagious,
   contaminated, contemptible, corroding, corrosive, corrupting,
   corruptive, counterproductive, crude, cussed, damaging, deadly,
   deleterious, despicable, destructive, detestable, detrimental,
   disadvantageous, disgusting, disserviceable, distressing,
   envenomed, execrable, fecal, fetid, forbidding, foul, frowsty,
   frowy, frowzy, fulsome, funky, fusty, gamy, graveolent, gross,
   harmful, hateful, heinous, high, hurtful, icky, ignoble,
   ill-smelling, infectious, infective, iniquitous, injurious,
   insalubrious, insanitary, invidious, lethal, loathsome, maggoty,
   malefic, maleficent, malevolent, malicious, malign, malignant,
   malodorous, mawkish, mean, mephitic, miasmal, miasmatic, miasmic,
   mildewed, mildewy, mischievous, moldy, morbific, musty, nasty,
   nauseant, nauseating, nauseous, nidorous, noisome, objectionable,
   obnoxious, obscene, odious, odorous, offensive, olid, ominous,
   ornery, overripe, pathogenic, peccant, pernicious, pestiferous,
   pestilent, pestilential, poisonous, polluted, prejudicial, putrid,
   rancid, rank, reasty, reasy, rebarbative, reechy, reeking, reeky,
   repellent, repugnant, repulsive, revolting, rotten, scatheful,
   septic, sickening, sickly, smellful, smelling, smelly, spoiled,
   stenchy, stinking, strong, stuffy, sulfurous, tainted, toxic,
   toxicant, toxiferous, unhealthful, unhealthy, unhygienic,
   unsanitary, unwholesome, venenate, veneniferous, venenous,
   venomous, vicious, vile, virulent, vomity, weevily, wicked,
   yucky