The Collaborative International Dictionary of English v.0.48:

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.
      [1913 Webster]

            Peace, you mumbling fool.             --Shak.
      [1913 Webster]

            A wrinkled hag, with age grown double,
            Picking dry sticks, and mumbling to herself.
                                                  --Otway.
      [1913 Webster]

   2. To chew something gently with closed lips.
      [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

Mumbling \Mum"bling\, a.
   Low; indistinct; inarticulate. -- Mum"bling*ly, adv.
   [1913 Webster]

WordNet (r) 3.0 (2006):

mumbling
    n 1: indistinct enunciation
    2: ineffectual chewing (as if without teeth) [syn: mumbling,
       gumming]