To hit someone or something with a dull thudding sound, or the sound itself. In cycling/running, refers to sudden energy depletion or 'hitting the wall.'
Imitative word from the early 20th century, mimicking the sound of a dull impact. The athletic meaning developed later as slang for the sudden fatigue when glycogen stores are depleted.
The word 'bonk' is a perfect example of onomatopoeia - it sounds exactly like what it describes. The athletic usage cleverly extends this 'hitting' metaphor to describe hitting an invisible wall of exhaustion, making an internal experience tangible through sound.
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