The Collaborative International Dictionary of English v.0.48:

Grimly \Grim"ly\, adv.
   In a grim manner; fiercely. --Shak.
   [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

Grimly \Grim"ly\, a.
   Grim; hideous; stern. [R.]
   [1913 Webster]

         In glided Margaret's grimly ghost,
         And stood at William's feet.             --D. Mallet.
   [1913 Webster]

WordNet (r) 3.0 (2006):

grimly
    adv 1: in a grim implacable manner; "he was grimly satisfied"

Moby Thesaurus II by Grady Ward, 1.0:

78 Moby Thesaurus words for "grimly":
   adamantly, appallingly, austerely, banefully, bleakly, decorously,
   demurely, direly, dismally, dourly, dreadfully, drearily,
   earnestly, firmly, formally, frowningly, funereally, gloomily,
   gloweringly, glumly, gravely, grumly, harshly, heavily, hideously,
   horribly, horridly, immovably, immutably, implacably, inclemently,
   inexorably, inflexibly, intransigently, irreconcilably, loweringly,
   moodily, mopingly, morosely, mumpishly, obdurately, relentlessly,
   rigidly, rigorously, roughly, ruggedly, sadly, scowlingly,
   sedately, seriously, severely, shockingly, sober-mindedly, soberly,
   sobersidedly, solemnly, somberly, sombrously, staidly, sternly,
   stiffly, strictly, stringently, sulkily, sullenly, surlily,
   terribly, thoughtfully, toughly, tragically, unalterably,
   unbendingly, unchangeably, uncompromisingly, unregenerately,
   unrelentingly, unsparingly, unyieldingly