Figuratively we use this phrase to indicate that something is so obvious it shouldn't be missed, like a nose. But what's the origin?  Read about it here . 

While there are two different potential origins for this phrase one seems more plausible than the other. It seems that holding someone's nose to the grindstone was originally a form of punishment. And it wasn't literal, it was a figure of speech to indicate someone had been strapped to a workbench so that they had to work continuously. ( source ) Today we use it in a figurative way to say someone is working hard and without breaks.

Comes from the ninth-century Ireland. When the Danes conquered the Irish, they imposed an exorbitant Nose Tax on the island's inhabitants. They took a census (by counting noses) and levied oppressive sums on their victims, forcing them to pay by threatening to have their noses actually slit. Paying the tax was "paying through the nose." ( source ) The phrase today doesn't have much to do with taxes but it does mean that someone is paying a ridiculous price for something.