The Collaborative International Dictionary of English v.0.48:
Grunt \Grunt\ (gr[u^]nt), n.
1. A deep, guttural sound, as of a hog.
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.
3. A U. S. infantryman; -- used especially of those fighting
in the war in Vietnam. [slang]