The Collaborative International Dictionary of English v.0.48:

Grim \Grim\ (gr[i^]m), a. [Compar. Grimmer (-m[~e]r); superl.
   Grimmest (-m[e^]st).] [AS. grim; akin to G. grimm, equiv.
   to G. & D. grimmig, Dan. grim, grum, Sw. grym, Icel. grimmr,
   G. gram grief, as adj., hostile; cf. Gr. ?, a crushing sound,
   ? to neigh.]
   Of forbidding or fear-inspiring aspect; fierce; stern; surly;
   cruel; frightful; horrible.
   [1913 Webster]

         Whose grim aspect sets every joint a-shaking. --Shak.
   [1913 Webster]

         The ridges of grim war.                  --Milton.

   Syn: Syn.-- Fierce; ferocious; furious; horrid; horrible;
        frightful; ghastly; grisly; hideous; stern; sullen;
        sour.
        [1913 Webster]

WordNet (r) 3.0 (2006):

grim
    adj 1: not to be placated or appeased or moved by entreaty;
           "grim determination"; "grim necessity"; "Russia's final
           hour, it seemed, approached with inexorable certainty";
           "relentless persecution"; "the stern demands of
           parenthood" [syn: grim, inexorable, relentless,
           stern, unappeasable, unforgiving, unrelenting]
    2: shockingly repellent; inspiring horror; "ghastly wounds";
       "the grim aftermath of the bombing"; "the grim task of
       burying the victims"; "a grisly murder"; "gruesome evidence
       of human sacrifice"; "macabre tales of war and plague in the
       Middle ages"; "macabre tortures conceived by madmen" [syn:
       ghastly, grim, grisly, gruesome, macabre, sick]
    3: harshly ironic or sinister; "black humor"; "a grim joke";
       "grim laughter"; "fun ranging from slapstick clowning ... to
       savage mordant wit" [syn: black, grim, mordant]
    4: harshly uninviting or formidable in manner or appearance; "a
       dour, self-sacrificing life"; "a forbidding scowl"; "a grim
       man loving duty more than humanity"; "undoubtedly the
       grimmest part of him was his iron claw"- J.M.Barrie [syn:
       dour, forbidding, grim]
    5: filled with melancholy and despondency ; "gloomy at the
       thought of what he had to face"; "gloomy predictions"; "a
       gloomy silence"; "took a grim view of the economy"; "the
       darkening mood"; "lonely and blue in a strange city";
       "depressed by the loss of his job"; "a dispirited and
       resigned expression on her face"; "downcast after his
       defeat"; "feeling discouraged and downhearted" [syn:
       gloomy, grim, blue, depressed, dispirited,
       down(p), downcast, downhearted, down in the mouth,
       low, low-spirited]
    6: causing dejection; "a blue day"; "the dark days of the war";
       "a week of rainy depressing weather"; "a disconsolate winter
       landscape"; "the first dismal dispiriting days of November";
       "a dark gloomy day"; "grim rainy weather" [syn: blue,
       dark, dingy, disconsolate, dismal, gloomy, grim,
       sorry, drab, drear, dreary]

Moby Thesaurus II by Grady Ward, 1.0:

293 Moby Thesaurus words for "grim":
   Spartan, Spartanic, adamant, adamantine, affording no hope,
   alarming, anguished, anxious, apathetic, appalling, astounding,
   astringent, atrocious, austere, authoritarian, awe-inspiring,
   awesome, awful, baneful, barbarous, barfy, beastly, beetle-browed,
   black, black-browed, bleak, bloodthirsty, bored, brutal, brutish,
   bum, cast-iron, certain, cheerless, cheesy, cold, comfortless,
   crappy, creepy, cruel, crummy, dark, dead set, decided, decorous,
   dejected, demanding, demure, depressed, despairing, desperate,
   despondent, determined, dire, direful, dirty, disconsolate,
   discontented, disgusted, dismal, dogged, dour, dread, dreaded,
   dreadful, drear, drearisome, dreary, dumpish, earnest, eerie, evil,
   evil-favored, evil-looking, exacting, exigent, fell, feral,
   ferocious, fiendish, fierce, firm, fixed, flagitious, flinty,
   forbidding, foreboding, forlorn, formal, formidable, frightening,
   frightful, frowning, funebrial, funereal, ghastly, ghoulish,
   gloomy, glowering, glum, godawful, goshawful, grave, gray,
   grim-faced, grim-visaged, grisly, grotesque, gruesome, grum, hairy,
   hard, hard-core, hard-favored, hard-featured, hard-visaged,
   harrowing, harsh, headstrong, heartless, heinous, hideous,
   homicidal, hopeless, horrendous, horrible, horrid, horrific,
   horrifying, humorless, icky, ill-favored, ill-featured,
   ill-looking, immovable, immutable, implacable, impliable,
   in bad humor, in despair, inclement, inelastic, inevitable,
   inexorable, infestive, inflexible, inhuman, inhumane, iniquitous,
   intractable, intransigent, iron, irreconcilable, joyless,
   loathsome, long-faced, lowering, lurid, macabre, melancholy,
   merciless, meticulous, mirthless, miserable, monstrous, moodish,
   moody, mopey, moping, mopish, morbid, morose, mortal, mumpish,
   murderous, nauseated, nauseous, obdurate, obstinate, offensive,
   ominous, out of humor, out of sorts, pitiless, pleasureless,
   prey to malaise, punk, putrid, redoubtable, relentless, repelled,
   repugnant, repulsive, resolute, revolted, revolting, rigid,
   rigorous, rock-ribbed, rotten, rough, rugged, ruthless, sad,
   saturnine, savage, schrecklich, scowling, sedate, serious, set,
   severe, shitty, shocking, sickened, sinister, sober, sober-minded,
   sobersided, solemn, somber, sombrous, sorry, sorryish, staid,
   steadfast, steely, stern, stiff, stinking, stinky, stone-faced,
   stony, straight-faced, strict, stringent, stubborn,
   suffering angst, sulky, sullen, surly, terrible, terrific,
   terrifying, thoughtful, tough, tragic, tremendous, triste,
   truculent, unaffected, unalterable, unappeasable, unbending,
   unchangeable, uncheerful, uncheery, uncompromising, uneasy,
   unflinching, unforgiving, unfulfilled, ungentle, ungiving,
   ungratified, unhappy, unhopeful, unjoyful, unmirthful, unmoved,
   unmoving, unprepossessing, unquiet, unrelenting, unsatisfied,
   unsmiling, unsparing, unspeakable, unwavering, unyielding, vicious,
   vindictive, violent, vomity, weariful, wearisome, weary, weighty,
   wicked, without hope, wolfish, wretched, yecchy