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.]