Source : Webster's Revised Unabridged Dictionary (1913)

Grog \Grog\, n. [So named from ``Old Grog'' a nickname given to
   Admiral Vernon, in allusion to his wearing a grogram cloak in
   foul weather. He is said to have been the first to dilute the
   rum of the sailors (about 1745).]
   A mixture of spirit and water not sweetened; hence, any
   intoxicating liquor.

   {Grog blossom}, a redness on the nose or face of persons who
      drink ardent spirits to excess. [Collog.]

Source : WordNet®

grog
     n : rum cut with water