The Collaborative International Dictionary of English v.0.48:

Slag \Slag\ (sl[a^]g), n. [Sw. slagg, or LG. slacke, whence G.
   schlacke; originally, perhaps, the splinters struck off from
   the metal by hammering. See Slay, v. t.]
   1. The dross, or recrement, of a metal; also, vitrified
      cinders.
      [1913 Webster]

   2. The scoria of a volcano.
      [1913 Webster]

   3. (Metal.) A product of smelting, containing, mostly as
      silicates, the substances not sought to be produced as
      matte or metal, and having a lower specific gravity than
      the latter; -- called also, esp. in iron smelting,
      cinder. The slag of iron blast furnaces is essentially
      silicate of calcium, magnesium, and aluminium; that of
      lead and copper smelting furnaces contains iron.
      [Webster 1913 Suppl.]

   Slag furnace, or Slag hearth (Metal.), a furnace, or
      hearth, for extracting lead from slags or poor ore.

   Slag wool, mineral wool. See under Mineral.
      [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

Slag \Slag\, v. i. & t. [imp. & p. p. Slagged; p. pr. & vb. n.
   Slagging.] (Metal.)
   To form, or form into, a slag; to agglomerate when heated
   below the fusion point.
   [Webster 1913 Suppl.]

WordNet (r) 3.0 (2006):

slag
    n 1: the scum formed by oxidation at the surface of molten
         metals [syn: slag, scoria, dross]
    v 1: convert into slag

Moby Thesaurus II by Grady Ward, 1.0:

81 Moby Thesaurus words for "slag":
   alluvion, alluvium, ash, ashes, bones, brand, calx, carbon, chaff,
   charcoal, cinder, clinker, coal, coke, coom, culm, deadwood,
   deposition, deposits, diluvium, dishwater, draff, dregs, dross,
   dust, ember, feces, filings, froth, fume, garbage, gash, grounds,
   hogwash, husks, lava, leavings, lees, loess, moraine, offal,
   offscourings, offscum, orts, parings, potsherds, precipitate,
   precipitation, rags, raspings, reek, refuse, scoria, scourings,
   scrap iron, scraps, scum, sediment, settlings, shards, shavings,
   silt, sinter, slack, slop, slops, smoke, smudge, smut, soot,
   stubble, sublimate, sullage, sweepings, swill, tares, wastage,
   waste, waste matter, wastepaper, weeds