Definitions of grunt:

• noun:   medium-sized tropical marine food fishes that utter a grunting sound when caught
• noun:   the short low gruff noise of the kind made by pigs
• noun:   an unskilled or low-ranking soldier or other worker

  Example: "Infantrymen in Vietnam were called grunts"

• verb:   issue a grunting, low, animal-like noise

  Example: "He grunted his reluctant approval"