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.
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         Who would fardels bear,
         To grunt and sweat under a weary life.   --Shak.
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   Grunting ox (Zool.), the yak.
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