Source : Webster's Revised Unabridged Dictionary (1913)

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

            Raise this strength, and sicken that to death.
                                                  --Prior.

   2. To make qualmish; to nauseate; to disgust; as, to sicken
      the stomach.

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

Sickening \Sick"en*ing\, a.
   Causing sickness; specif., causing surfeit or disgust;
   nauseating. -- {Sick"en*ing*ly}, adv.

Source : WordNet®

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