The Collaborative International Dictionary of English v.0.48:

Grum \Grum\ (gr[u^]m), a. [Cf. Dan. grum furious, Sw. grym, AS.
   gram, and E. grim, and grumble. [root]35.]
   1. Morose; severe of countenance; sour; surly; glum; grim.
      "Nick looked sour and grum." --Arbuthnof.
      [1913 Webster]

   2. Low; deep in the throat; guttural; rumbling; as, a grum
      voice.
      [1913 Webster]

Moby Thesaurus II by Grady Ward, 1.0:

29 Moby Thesaurus words for "grum":
   beetle-browed, black, black-browed, brooding, broody, chapfallen,
   crestfallen, dark, dejected, dour, dumpish, frowning, glowering,
   glum, grim, long-faced, lowering, melancholy, moodish, moody,
   mopey, moping, mopish, morose, mumpish, scowling, sulky, sullen,
   surly