Source : Webster's Revised Unabridged Dictionary (1913)

Mumble \Mum"ble\, v. t. [imp. & p. p. {Mumbled}; p. pr. & vb. n.
   {Mumbling}.] [OE. momelen; cf. D. mompelen, mommelen, G.
   mummelen, Sw. mumla, Dan. mumle. Cf. {Mum}, a., {Mumm},
   {Mump}, v.]
   1. To speak with the lips partly closed, so as to render the
      sounds inarticulate and imperfect; to utter words in a
      grumbling indistinct manner, indicating discontent or
      displeasure; to mutter.

            Peace, you mumbling fool.             --Shak.

            A wrinkled hag, with age grown double, Picking dry
            sticks, and mumbling to herself.      --Otway.

   2. To chew something gently with closed lips.