The Collaborative International Dictionary of English v.0.48:
Killing \Kill"ing\, a. Literally, that kills; having power to kill; fatal; in a colloquial sense, conquering; captivating; irresistible. -- Kill"ing*ly, adv. [1913 Webster] Those eyes are made so killing. --Pope. [1913 Webster] Nothing could be more killingly spoken. --Milton. [1913 Webster]WordNet (r) 3.0 (2006):
killingly adv 1: in a very humorous manner [syn: killingly, sidesplittingly]