The Collaborative International Dictionary of English v.0.48:

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.
   [1913 Webster]

         Who would fardels bear,
         To grunt and sweat under a weary life.   --Shak.
   [1913 Webster]

   Grunting ox (Zool.), the yak.
      [1913 Webster]

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]

The Collaborative International Dictionary of English v.0.48:

Redmouth \Red"mouth`\ (-mouth`), n. (Zool.)
   Any one of several species of marine food fishes of the genus
   Diabasis, or Haemulon, of the Southern United States,
   having the inside of the mouth bright red. Called also
   flannelmouth, and grunt.
   [1913 Webster]

WordNet (r) 3.0 (2006):

grunt
    n 1: the short low gruff noise of the kind made by hogs [syn:
         grunt, oink]
    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 grunting
       sounds when caught
    v 1: issue a grunting, low, animal-like noise; "He grunted his
         reluctant approval"

Moby Thesaurus II by Grady Ward, 1.0:

112 Moby Thesaurus words for "grunt":
   Zouave, air a grievance, animal noise, bark, barking, bawl, beef,
   bellow, bellyache, bersagliere, birdcall, bitch, blare, blat,
   blubber, boom, bray, breathe, buzz, cackle, call, carabineer,
   chant, chasseur, chirp, clamor, clang, complain, coo, crab, croak,
   crow, cry, dogface, doughfoot, drawl, exclaim, expert rifleman,
   flute, foot soldier, footslogger, fret, fret and fume, fusileer,
   fuss, gasp, grenadier, gripe, groan, grouch, grouse, growl,
   grumble, gruntle, hiss, holler, howl, howling, infantryman, keen,
   kick, light infantryman, lilt, lodge a complaint, marksman,
   mating call, mumble, murmur, musketeer, mutter, note, oink,
   paddlefoot, pant, pipe, raise a howl, register a complaint, rifle,
   rifleman, roar, rumble, scold, scream, screech, sharpshooter,
   shriek, sibilate, sigh, sing, snap, snarl, sniper, snort, sob,
   squall, squawk, squeal, stridulation, take on, thunder, trumpet,
   twang, ululation, wail, warble, whine, whisper, woodnote, yap,
   yawp, yell, yelp