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]