= millstone (1)
a flat circular stone or abrasive disc that revolves on an axle and is used for grinding, polishing, sharpening shaping, etc
keep one's nose to the grindstone
to continue working conscientiously
[recorded in the form hold somebody's nose to the grindstone from the 16th cent., although in its present form it is 19th cent. The grindstone is an image for unrelenting work]