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.
      [1913 Webster]

   2. Rust of metals. [R.] --Addison.
      [1913 Webster]

   3. Waste matter; any worthless matter separated from the
      better part; leavings; dregs; refuse.
      [1913 Webster]

            All world's glory is but dross unclean. --Spenser.
      [1913 Webster]

            At the devil's booth are all things sold,
            Each ounce of dross coats its ounce of gold.
                                                  --Lowell.
      [1913 Webster]

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]

Moby Thesaurus II by Grady Ward, 1.0:

44 Moby Thesaurus words for "dross":
   alluvion, alluvium, ash, ashes, brand, calx, carbon, charcoal,
   cinder, clinker, coal, coke, coom, deposition, deposits, diluvium,
   draff, dregs, ember, feces, froth, fume, grounds, lava, lees,
   loess, moraine, offscum, precipitate, precipitation, reek, scoria,
   scum, sediment, settlings, silt, sinter, slag, smoke, smudge, smut,
   soot, sublimate, sullage



Easton's 1897 Bible Dictionary:

Dross
   the impurities of silver separated from the one in the process
   of melting (Prov. 25:4; 26:23; Ps. 119:119). It is also used to
   denote the base metal itself, probably before it is smelted, in
   Isa. 1:22, 25.