The Collaborative International Dictionary of English v.0.48:
Appalling \Ap*pall"ing\, a. Such as to appall; as, an appalling accident. -- Ap*pall"ing*ly, adv. [1913 Webster]WordNet (r) 3.0 (2006):
appallingly adv 1: to an appalling extent; "the prisoners were appallingly thin"