Source : Webster's Revised Unabridged Dictionary (1913)

Grunt \Grunt\ (gr[u^]nt), v. t. [imp. & p. p. {Grunted}; p. pr.
   & vb. n. {Grunting}.] [OE. grunten; akin to As. grunian, G.
   grunzen, Dan. grynte, Sw. grymta; all prob. of imitative; or
   perh. akin to E. groan.]
   To make a deep, short noise, as a hog; to utter a short groan
   or a deep guttural sound.

         Who would fardels bear, To grunt and sweat under a
         weary life.                              --Shak.

   {Grunting ox} (Zo["o]l.), the yak.

Grunt \Grunt\, n.
   1. A deep, guttural sound, as of a hog.

   2. (Zo["o]l.) Any one of several species of American food
      fishes, of the genus {H[ae]mulon}, allied to the snappers,
      as, the black grunt ({A. Plumieri}), and the redmouth
      grunt ({H. 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.

Source : WordNet®

grunt
     n 1: the short low gruff noise of the kind made by pigs
     2: an unskilled or low-ranking soldier or other worker;
        "infantrymen in Vietnam were called grunts"; "he went from
        grunt to chairman in six years"
     3: medium-sized tropical marine food fishes that utter a
        grunting sound when caught
     v : issue a grunting, low, animal-like noise; "He grunted his
         reluctant approval"