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.
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            You that scum the molten lead.        --Dryden &
                                                  Lee.
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   2. To sweep or range over the surface of. [Obs.]
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            Wandering up and down without certain seat, they
            lived by scumming those seas and shores as pirates.
                                                  --Milton.
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