The Collaborative International Dictionary of English v.0.48:

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.
      [1913 Webster]

            You that scum the molten lead.        --Dryden &
                                                  Lee.
      [1913 Webster]

   2. To sweep or range over the surface of. [Obs.]
      [1913 Webster]

            Wandering up and down without certain seat, they
            lived by scumming those seas and shores as pirates.
                                                  --Milton.
      [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

Scumming \Scum"ming\, n.
   (a) The act of taking off scum.
   (b) That which is scummed off; skimmings; scum; -- used
       chiefly in the plural.
       [1913 Webster]