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}.