Source : Webster's Revised Unabridged Dictionary (1913)

Scumming \Scum"ming\, n.
   (a) The act of taking off scum.
   (b) That which is scummed off; skimmings; scum; -- used
       chiefly in the plural.

Scum \Scum\, v. t. [imp. & p. p. {Scummed}; p. pr. & vb. n.
   {Scumming}.]
   1. To take the scum from; to clear off the impure matter from
      the surface of; to skim.

            You that scum the molten lead.        --Dryden &
                                                  Lee.

   2. To sweep or range over the surface of. [Obs.]

            Wandering up and down without certain seat, they
            lived by scumming those seas and shores as pirates.
                                                  --Milton.

Source : WordNet®

scum
     n 1: worthless people [syn: {trash}]
     2: a film of impurities or vegetation that can form on the
        surface of a liquid
     [also: {scumming}, {scummed}]

scumming
     See {scum}