The Collaborative International Dictionary of English v.0.48:

Whetstone \Whet"stone`\, n. [AS. hwetst[=a]n.]
   A piece of stone, natural or artificial, used for whetting,
   or sharpening, edge tools.
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         The dullness of the fools is the whetstone of the wits.
                                                  --Shak.
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         Diligence is to the understanding as the whetstone to
         the razor.                               --South.
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   Note: Some whetstones are used dry, others are moistened with
         water, or lubricated with oil.
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   To give the whetstone, to give a premium for extravagance
      in falsehood. [Obs.]
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