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]