Grindstone /(?)/

Grind·stone

Grindstone

n.
  1. A flat, circular stone, revolving on an axle, for grinding or sharpening tools, or shaping or smoothing objects.
    They might be ashamed, for lack of courage, to suffer the Lacedæmonians to hold their noses to the grindstone.
    — Sir T. North.

Phrases & Compounds
To hold 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
to oppress one; to cause one to work hard and steadily.
To keep one's nose to the grindstone
to continue at a long and arduous task; to apply oneself steadily to one's duties.