The Collaborative International Dictionary of English v.0.48:

Sickening \Sick"en*ing\, a.
   Causing sickness; specif., causing surfeit or disgust;
   nauseating. -- Sick"en*ing*ly, adv.
   [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

Sicken \Sick"en\, v. t. [imp. & p. p. Sickened; p. pr. & vb.
   n. Sickening.]
   1. To make sick; to disease.
      [1913 Webster]

            Raise this strength, and sicken that to death.
                                                  --Prior.
      [1913 Webster]

   2. To make qualmish; to nauseate; to disgust; as, to sicken
      the stomach.
      [1913 Webster]

   3. To impair; to weaken. [Obs.] --Shak.
      [1913 Webster]

WordNet (r) 3.0 (2006):

sickening
    adj 1: causing or able to cause nausea; "a nauseating smell";
           "nauseous offal"; "a sickening stench" [syn:
           nauseating, nauseous, noisome, queasy,
           loathsome, offensive, sickening, vile]

Moby Thesaurus II by Grady Ward, 1.0:

60 Moby Thesaurus words for "sickening":
   abhorrent, abominable, bad, barfy, base, beastly, below contempt,
   beneath contempt, brackish, cloying, contemptible, crude,
   despicable, detestable, disgusting, execrable, fetid, forbidding,
   foul, fulsome, gross, hateful, heinous, high, icky, ignoble,
   loathsome, maggoty, malodorous, mawkish, mephitic, miasmal,
   miasmic, nasty, nauseant, nauseating, nauseous, noisome, noxious,
   objectionable, obnoxious, obscene, odious, offensive, overripe,
   poisonous, rancid, rank, rebarbative, repellent, repugnant,
   repulsive, revolting, rotten, spoiled, stinking, vile, vomity,
   weevily, yucky