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]WordNet (r) 3.0 (2006):
sickeningly adv 1: in a disgusting manner or to a disgusting degree; "the beggar was disgustingly filthy" [syn: disgustingly, distastefully, revoltingly, sickeningly]