Source : Webster's Revised Unabridged Dictionary (1913)

Killing \Kill"ing\, a.
   Literally, that kills; having power to kill; fatal; in a
   colloquial sense, conquering; captivating; irresistible. --
   {Kill"ing*ly}, adv.

         Those eyes are made so killing.          --Pope.

         Nothing could be more killingly spoken.  --Milton.

Source : WordNet®

killingly
     adv : in a very humorous manner [syn: {sidesplittingly}]