whet·stone /ˈhwɛtˌston, ˈwɛt-/
  磨石
  Whet·stone n.  A piece of stone, natural or artificial, used for whetting, or sharpening, edge tools.
     The dullness of the fools is the whetstone of the wits.   --Shak.
     Diligence is to the understanding as the whetstone to the razor.   --South.
  Note: ☞ Some whetstones are used dry, others are moistened with water, or lubricated with oil.
  To give the whetstone, to give a premium for extravagance in falsehood.  [Obs.]
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  whetstone
       n : a flat stone for sharpening edged tools or knives