Source : Webster's Revised Unabridged Dictionary (1913)

Slag \Slag\, 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.

   2. The scoria of a volcano.

   {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}.

Slag \Slag\, n. (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.

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.

Source : WordNet®

slag
     n : the scum formed by oxidation at the surface of molten metals
         [syn: {scoria}, {dross}]
     [also: {slagging}, {slagged}]