The present participle of 'conk,' meaning to hit or strike, especially used in 'conk out' meaning to stop working or fall asleep suddenly.
From onomatopoeia 'conk'—the word mimics the sound of a hard blow or impact. The 'out' version came into popular use in the 20th century, especially in American slang.
The phrase 'conk out' became super popular during WWII when pilots would say their engines 'conked out,' and soldiers spread the slang everywhere—it's pure onomatopoeia that English speakers intuitively understood!
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