grindstone (n.) Look up grindstone at
early 13c. "millstone," from grind (v.) in sense of "sharpen" + stone (n.); meaning "revolving stone disc used for sharpening, etc." is from c. 1400. Phrase nose to the grindstone in use by 1530s; originally to get control of another and treat him harshly:
This Text holdeth their noses so hard to the grindstone, that it clean disfigureth their Faces. [John Frith, "Mirror to know Thyself," 1532]
The phrase's main modern (reflexive) sense of "working hard" is from 1828.