to make a low, rough sound in the throat, like the noise a pig makes or a person makes when lifting something heavy.
From Old English 'grunnettan' (to grunt). Onomatopoetic, it imitates the actual sound made and has been used since around 1400.
Grunts are one of the oldest sounds humans make—it's onomatopoeia that goes back to Middle English, and pigs and humans both grunt for basically the same reason: effort and communication!
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