Source : Webster's Revised Unabridged Dictionary (1913)

Scumble \Scum"ble\, v. t. [imp. & p. p. {Scumbled}; p. pr. & vb.
   n. {Scumbling}.] [Freq. of scum. [root] 158.] (Fine Arts)
   To cover lighty, as a painting, or a drawing, with a thin
   wash of opaque color, or with color-crayon dust rubbed on
   with the stump, or to make any similar additions to the work,
   so as to produce a softened effect.

Scumbling \Scum"bling\, n.
   1. (Fine Arts)
      (a) A mode of obtaining a softened effect, in painting and
          drawing, by the application of a thin layer of opaque
          color to the surface of a painting, or part of the
          surface, which is too bright in color, or which
          requires harmonizing.
      (b) In crayon drawing, the use of the stump.

   2. The color so laid on. Also used figuratively.

            Shining above the brown scumbling of leafless
            orchards.                             --L. Wallace.