Hard at work.

Examples

• "Nose to the grindstone, he was up all night."

Etymology

From the literal action of intensely working a grindstone, whether powered by a treadle or waterwheel. The expression initially implied punishment or abusive management, forcing the worker into intense work, and was used in the anonymous 1557 translation of Erasmus's Merry Dialogue as a hyperbolic punishment threatened for an abusive husband. It was later adapted to forcing oneself into similarly intense effort.

Sources

• Wiktionary