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]