Source : Webster's Revised Unabridged Dictionary (1913)

Dross \Dross\, n. [AS. dros, fr. dre['o]san to fall. See
   {Dreary}.]
   1. The scum or refuse matter which is thrown off, or falls
      from, metals in smelting the ore, or in the process of
      melting; recrement.

   2. Rust of metals. [R.] --Addison.

   3. Waste matter; any worthless matter separated from the
      better part; leavings; dregs; refuse.

            All world's glory is but dross unclean. --Spenser.

            At the devil's booth are all things sold, Each ounce
            of dross coats its ounce of gold.     --Lowell.

Source : WordNet®

dross
     n 1: worthless material that should be removed; "there were
          impurities in the water" [syn: {impurity}]
     2: the scum formed by oxidation at the surface of molten metals
        [syn: {slag}, {scoria}]