Grindstone \Grind"stone`\, n.
A flat, circular stone, revolving on an axle, for grinding or
sharpening tools, or shaping or smoothing objects.
[1913 Webster]
{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}
(a) to oppress one; to cause one to work hard and steadily.
(b) 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.
[1913 Webster +PJC]
They might be ashamed, for lack of courage, to
suffer the Laced[ae]monians to hold their noses to
the grindstone. --Sir T.
North.
[1913 Webster]