Source : Webster's Revised Unabridged Dictionary (1913)

Grindstone \Grind"stone`\, n.
   A flat, circular stone, revolving on an axle, for grinding or
   sharpening tools, or shaping or smoothing objects.

   {To} {hold, pat, or bring} {one's nose to the grindstone}, to
      oppress one; to keep one in a condition of servitude.

            They might be ashamed, for lack of courage, to
            suffer the Laced[ae]monians to hold their noses to
            the grindstone.                       --Sir T.
                                                  North.

Source : WordNet®

grindstone
     n : a revolving stone shaped like a disk; used to grind or
         sharpen or polish edge tools