What does "grindstone" mean?

Grindstone: 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 Lacedæmonians to hold their noses to the grindstone. Sir T. North.