To make something sharp or sharper, or to make something more intense, focused, or clear.
From Old English 'scearp' meaning 'sharp,' combined with the verb-making suffix '-en.' The metaphorical meaning (to make clear or intense) evolved over centuries.
The metaphorical use of 'sharpen' reveals how humans think—we describe mental clarity using physical sharpness, which is why we also say arguments are 'sharp,' wit is 'keen,' and thoughts are 'acute.' Language tells us our ancestors saw the mind as a blade!
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