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

For further exploring for "grimly" in Webster Dictionary Online