nose to the grindstone
Users lay on (often strapped to) a plank above the grindstone. First attested 1532, from John Frith's A Mirror to know Thyself: "This Text holdeth their noses so hard to the grindstone, that it clean disfigureth their faces." Best guess: a water wheel "water mill" was used to grind wheat. When too much wheat is added or the (adjustable) millstone tolerance is incorrect it would burn the wheat and this was detected by smell. So it means pay strict attention to your work so you catch something like this.
- (idiomatic, after keep, put, have, hold (one's)) used to form idioms meaning "to work hard, or focus heavily on work."
- If he'd keep his nose to the grindstone a bit more, he could be a pretty good student.
- ^ The meaning and origin of the expression: Keep your nose to the grindstone, the Phrase Finder