From Websters 1828 Dictionary (link)

Dross DROSS, n. [G.]
1. The recrement or despumation of metals; the scum or extraneous matter of metals, thrown off in the process of melting.
2. Rust; crust of metals; an incrustation formed on metals by oxydation.
3. Waste matter; refuse; any worthless matter separated from the better part; impure matter.
The worlds glory is but dross unclean.

From WordNet (r) 2.1 (2005) (link)

  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]

From Merriam-Webster's Collegiate Dictionary, 11th Edition (2003) (link)

  dross noun
   Etymology: Middle English dros, from Old English drōs
   dregs Date: before 12th century 1.  the scum that forms on
   the surface of molten metal 2.  waste or foreign matter ;
   impurity 3.  something that is base, trivial, or inferior •
   drossy adjective
  
  

From Oxford English Reference Dictionary (link)

dross
n.
1 rubbish, refuse.
2 a the scum separated from metals in melting. b foreign matter mixed with anything; impurities.
Derivatives:
drossy adj.
Etymology: OE dros: cf. MLG drosem, OHG truosana

From Collins COBUILD Advanced Learner's English Dictionary (link)

  dross
    If you describe something as dross, you mean that it is of very poor quality or has
    no value. (LITERARY)
    I go through phases where everything I write is just dross.
   = rubbish
   N-UNCOUNT [disapproval]
  
  
  

From English Explanatory Dictionary (link)

  dross
  drɔs n. 1 rubbish, refuse. 2 a the scum separated from metals
  in melting. b foreign matter mixed with anything; impurities. øødrossy
  adj. [OE dros: cf. MLG drosem, OHG truosana]
  
  
  

From Webster's Revised Unabridged Dictionary (1913) (link)

  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.

From International Standard Bible Encyclopedia (link)

  DROSS
  dros (sigh): The refuse of smelting of precious metal (Pr 25:4; 26:23);
  used figuratively of what is base or worthless (Isa 1:22,25; Eze 22:18,19;
  Ps 119:119).
  
  
  

From Easton's 1897 Bible Dictionary (link)

  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.
     

From Moby Thesaurus II by Grady Ward, 1.0 (link)

  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