The Collaborative International Dictionary of English v.0.48:

Dross \Dross\ (dr[o^]s; 115), 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.
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   2. Rust of metals. [R.] --Addison.
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   3. Waste matter; any worthless matter separated from the
      better part; leavings; dregs; refuse.
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            All world's glory is but dross unclean. --Spenser.
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            At the devil's booth are all things sold,
            Each ounce of dross coats its ounce of gold.
                                                  --Lowell.
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WordNet (r) 3.0 (2006):

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