Source : Webster's Revised Unabridged Dictionary (1913)

Scum \Scum\, v. i.
   To form a scum; to become covered with scum. Also used
   figuratively.

         Life, and the interest of life, have stagnated and
         scummed over.                            --A. K. H.
                                                  Boyd.

Scum \Scum\, n. [Of Scand. origin; cf. Dan. & Sw. skum, Icel.
   sk?m, LG. schum, D. schuim, OHG. sc?m, G. schaum; probably
   from a root meaning, to cover. [root]158. Cf. {Hide} skin,
   {Meerschaum}, {Skim}, v., {Sky}.]
   1. The extraneous matter or impurities which rise to the
      surface of liquids in boiling or fermentation, or which
      form on the surface by other means; also, the scoria of
      metals in a molten state; dross.

            Some to remove the scum it did rise.  --Spenser.

   2. refuse; recrement; anything vile or worthless.

            The great and innocent are insulted by the scum and
            refuse of the people.                 --Addison.

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}]