The Collaborative International Dictionary of English v.0.48:

Grunt \Grunt\ (gr[u^]nt), n.
   1. A deep, guttural sound, as of a hog.
      [1913 Webster]

   2. (Zool.) Any one of several species of American food
      fishes, of the genus Haemulon, allied to the snappers,
      as, the black grunt (Haemulon Plumieri), and the
      redmouth grunt (Haemulon aurolineatus), of the Southern
      United States; -- also applied to allied species of the
      genera Pomadasys, Orthopristis, and Pristopoma.
      Called also pigfish, squirrel fish, and grunter; --
      so called from the noise it makes when taken.
      [1913 Webster]

   3. A U. S. infantryman; -- used especially of those fighting
      in the war in Vietnam. [slang]
      [PJC]