An exclamation used to express cold or to shiver; imitative of the sound someone makes when they're shivering from cold.
Onomatopoeia—the word imitates the actual sound made when shivering or the vibration of teeth chattering in cold weather. It's been spelled various ways ('brrr,' 'br,' 'brrrr') and has been in English since at least the 1800s.
Onomatopoeia like 'brr' reveal how much language is baked into our bodies—it's not just random; your mouth naturally makes a 'brrr' sound when you shiver, so the word is literally your body expressing itself.
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