To make a low, rough sound in the throat, often showing effort, pain, or displeasure.
From Old English 'grunnettan,' which imitated the sound that pigs make. The word has stayed largely unchanged for over a thousand years because it's meant to sound like the actual noise.
Grunts is one of the oldest onomatopoeia in English—a word that literally sounds like what it means. Pigs have been grunting the same way for millennia, so we just borrowed their sound and made it our word for that noise.
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