The Collaborative International Dictionary of English v.0.48:

Grumble \Grum"ble\, v. i. [imp. & p. p. Grunbled; p. pr. & vb.
   n. Grumbling.] [Cf. LG. grummeln, grumen, D. grommelen,
   grommen, and F. grommeler, of German origin; cf. W. grwm,
   murmur, grumble, surly. [root]35. Cf. Grum, Grim.]
   1. To murmur or mutter with discontent; to make ill-natured
      complaints in a low voice and a surly manner.
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            L'Avare, not using half his store,
            Still grumbles that he has no more.   --Prior.
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   2. To growl; to snarl in deep tones; as, a lion grumbling
      over his prey.
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   3. To rumble; to make a low, harsh, and heavy sound; to
      mutter; as, the distant thunder grumbles.
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