The Collaborative International Dictionary of English v.0.48:

Gum \Gum\, v. t. [imp. & p. p. Gummed (g[u^]md); p. pr. & vb.
   n. Gumming.]
   1. To smear with gum; to close with gum; to unite or stiffen
      by gum or a gumlike substance; to make sticky with a
      gumlike substance.
      [1913 Webster]

            He frets like a gummed velvet.        --Shak.
      [1913 Webster]

   2. To chew with the gums, rather than with the teeth.
      [PJC]

   gum up
      (a) To block or clog (a conduit) with or as if with gum;
          as, to gum up the drainpipe.
      (b) to interfere with; to spoil. [Slang]
          [PJC]

WordNet (r) 3.0 (2006):

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