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]