A dull, heavy sound made when something solid hits something else, like a book dropping on the floor.
Possibly imitative in origin, appearing in English around the 1600s. The word mimics the actual sound it describes, making it onomatopoeia.
Onomatopoeia words like 'thud' are actually shorter versions of longer sounds—your brain picks out the most distinctive part of the noise and turns it into a word!
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