Source : Webster's Revised Unabridged Dictionary (1913)

Mordacious \Mor*da"cious\, a. [L. mordax, -acis, fr. mordere,
   morsum, to bite. See {Morsel}.]
   Biting; given to biting; hence, figuratively, sarcastic;
   severe; scathing. -- {Mor*da"cious*ly}, adv.

Source : WordNet®

mordaciously
     adv : in a mordacious manner; "she taunted him mordaciously"