The Collaborative International Dictionary of English v.0.48:

Monstrously \Mon"strous*ly\, adv.
   In a monstrous manner; unnaturally; extraordinarily; as,
   monstrously wicked. "Who with his wife is monstrously in
   love." --Dryden.
   [1913 Webster]

WordNet (r) 3.0 (2006):

monstrously
    adv 1: in a hideous manner; "her face was hideously disfigured
           after the accident" [syn: hideously, horridly,
           monstrously]
    2: in a terribly evil manner; "the child was heinously murdered"
       [syn: heinously, monstrously]
    3: in a grotesque manner; "behind the house lay two nude figures
       grotesquely bald, with deliberate knife-slashes marking their
       bodies" [syn: grotesquely, monstrously]

Moby Thesaurus II by Grady Ward, 1.0:

36 Moby Thesaurus words for "monstrously":
   abjectly, abominably, arrantly, atrociously, basely, contemptibly,
   despicably, eerily, execrably, fancifully, fantastically,
   flagrantly, foully, grossly, grotesquely, heinously, meanly,
   miserably, mysteriously, nefariously, obnoxiously, odiously,
   outlandishly, peculiarly, pettily, poorly, quaintly, queerly,
   scurvily, shabbily, shoddily, singularly, strangely,
   supernaturally, vilely, wretchedly