Source : Webster's Revised Unabridged Dictionary (1913)

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

Source : WordNet®

sickeningly
     adv : in a disgusting manner or to a disgusting degree; "the
           beggar was disgustingly filthy" [syn: {disgustingly}, {distastefully},
            {revoltingly}]