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.

Source : WordNet®

grunting
     adj : making short inarticulate guttural sounds as of effort
           expended; "ferries grunting on the river"; "grunting
           boxers"