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

U.S. Gazetteer Places (2000):

Grindstone-Rowes Run, PA -- U.S. Census Designated Place in Pennsylvania
   Population (2000):    1141
   Housing Units (2000): 530
   Land area (2000):     2.145976 sq. miles (5.558052 sq. km)
   Water area (2000):    0.000000 sq. miles (0.000000 sq. km)
   Total area (2000):    2.145976 sq. miles (5.558052 sq. km)
   FIPS code:            31594
   Located within:       Pennsylvania (PA), FIPS 42
   Location:             40.012106 N, 79.820708 W
   ZIP Codes (1990):     15442
   Note: some ZIP codes may be omitted esp. for suburbs.
   Headwords:
    Grindstone-Rowes Run, PA
    Grindstone-Rowes Run
    Grindstone, PA
    Grindstone