keep one's nose to the grindstone

Example: “If you keep your nose to the grindstone this semester, you'll graduate on time.”

Origin: The direction of this phrase has reversed over time, and the earlier version is considerably nastier. In its first recorded uses, from the 1500s, one person holds ANOTHER'S nose to the grindstone — an image of oppression, of keeping someone bent to unrelenting labor under threat — and the phrase belonged with expressions about grinding the faces of the poor rather than with expressions about diligence. The modern self-applied sense, where a person keeps their own nose down and works steadily, is the later development, and it converts what was a description of exploitation into something close to a virtue. The stone itself is most likely a tool-sharpening wheel, over which a worker must lean in very close to see the edge being ground.