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 Lacedæmonians to hold their noses to the grindstone. Sir T. North. [ 1913 Webster ]