Source : Webster's Revised Unabridged Dictionary (1913)

Coalsack \Coal"sack`\, n. [Coal + 2d sack.] (Astron.)
   Any one of the spaces in the Milky Way which are very black,
   owing to the nearly complete absence of stars; esp., the
   large space near the Southern Cross sometimes called the
   {Black Magellanic Cloud}.