The image it conjures is someone literally pressing their nose against a grindstone, which is a heavy stone used to sharpen tools. This implies a relentless and focused effort, often associated with manual labor or repetitive tasks.
Here are some examples of how the expression is used:
* "I've been working nose to the grindstone all week to meet the deadline."
* "He's been nose to the grindstone since he got that promotion."
* "It's time to get back to the grindstone and finish this project."
The phrase often implies a sense of dedication, determination, and perhaps even exhaustion, but it's not necessarily a negative connotation. It can also suggest a sense of pride in one's hard work.