What is "to keep one's nose to the grindstone"

Grindstone \Grind"stone`\, n. A flat, circular stone, revolving on an axle, for grinding or sharpening tools, or shaping or smoothing objects. To hold one's nose to the grindstone, To bring one's nose to the grindstone, to oppress one; to keep one in a condition of servitude. To put one's nose to the grindstone

1. to oppress one; to cause one to work hard and steadily.

2. to set oneself to a long and arduous task.

   To keep one's nose to the grindstone to continue at a long and arduous task; to apply oneself steadily to one's duties.

   They might be ashamed, for lack of courage, to suffer the Laced[ae]monians to hold their noses to the grindstone.
   --Sir T. North.