What does grindstone mean?

[Zoo]

English Language Tests, Intermediate level

ESL/EFL Test #381 "English Slang Idioms (112)", question 3

He knew that to succeed in the job he had, one had to keep his or her nose to the ......... from the time he or she got to the door until close of business.

(a) grindstone
(b) face
(c) mirror
(d) booking

English Language Tests, Intermediate level

ESL/EFL Test #381 "English Slang Idioms (112)", answer 3

He knew that to succeed in the job he had, one had to keep his or her nose to the grindstone from the time he or she got to the door until close of business.

Correct answer: (a) grindstone

Your answer was: correct
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what does mean grindstone?

Zoo

[Linda]

Hello Zoo,
Literally, a grindstone is: "A revolving stone disk used for grinding, polishing, or sharpening tools" but this idiom "keep your nose grindstone" means to work very hard.
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