To sharpen a blade or tool on a stone, or to stimulate someone's interest or appetite.
From Old English 'hwettan' meaning 'to sharpen.' The metaphorical sense of 'to stimulate' emerged around the 13th century, as sharpening came to represent making something more acute or keen.
Your appetite and a knife blade work similarly in language—both can be 'whetted' or made sharp and intense, which is why poets use the word metaphorically to describe desire becoming more pointed and focused.
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