The Collaborative International Dictionary of English v.0.48:
Withering \With"er*ing\, a. Tending to wither; causing to shrink or fade. -- With"er*ing*ly, adv. [1913 Webster]WordNet (r) 3.0 (2006):
witheringly adv 1: in a withering manner; "guns fired witheringly at the railroad cars"