The Collaborative International Dictionary of English v.0.48:

Grindstone \Grind"stone`\, n.
   A flat, circular stone, revolving on an axle, for grinding or
   sharpening tools, or shaping or smoothing objects.
   [1913 Webster]

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

   To put one's nose to the grindstone
   (a) to oppress one; to cause one to work hard and steadily.
   (b) to set oneself to a long and arduous task.

   To keep one's nose to the grindstone to continue at a long
      and arduous task; to apply oneself steadily to one's
      duties.
      [1913 Webster +PJC]

            They might be ashamed, for lack of courage, to
            suffer the Laced[ae]monians to hold their noses to
            the grindstone.                       --Sir T.
                                                  North.
      [1913 Webster]

WordNet (r) 3.0 (2006):

grindstone
    n 1: a revolving stone shaped like a disk; used to grind or
         sharpen or polish edge tools