Source : Webster's Revised Unabridged Dictionary (1913)

Grumble \Grum"ble\, v. i. [imp. & p. p. {Grunbled}; p. pr. & vb.
   n. {Grumbling}.] [Cf. LG. grummeln, grumman, 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.

            L'Avare, not using half his store, Still grumbles
            that he has no more.                  --Prior.

   2. To growl; to snarl in deep tones; as, a lion grumbling
      over his prey.

   3. To rumble; to make a low, harsh, and heavy sound; to
      mutter; as, the distant thunder grumbles.

Grumble \Grum"ble\, v. t.
   To express or utter with grumbling.

Grumble \Grum"ble\, n.
   1. The noise of one that grumbles.

   2. A grumbling, discontented disposition.

            A bad case of grumble.                --Mrs. H. H.
                                                  Jacksn.

Source : WordNet®

grumble
     n 1: a loud low dull continuous noise; "they heard the rumbling
          of thunder" [syn: {rumble}, {rumbling}, {grumbling}]
     2: a complaint uttered in a low and indistinct tone [syn: {grumbling},
         {murmur}, {murmuring}, {mutter}, {muttering}]

grumble
     v 1: show one's unhappiness or critical attitude; "He scolded
          about anything that he thought was wrong"; "We grumbled
          about the increased work load" [syn: {grouch}, {scold}]
     2: make complaining remarks or noises under one's breath; "she
        grumbles when she feels overworked" [syn: {murmur}, {mutter},
         {croak}, {gnarl}]
     3: to utter or emit low dull rumbling sounds; "he grumbled a
        rude response"; "Stones grumbled down the cliff" [syn: {growl},
         {rumble}]
     4: make a low noise; "rumbling thunder" [syn: {rumble}]