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 put one's nose to the grindstone
To keep one's nose to the grindstone
They might be ashamed, for lack of courage, to suffer the Lacedæmonians to hold their noses to the grindstone. Sir T. North. [ 1913 Webster ]