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]