keep one's nose to the grindstone
keep one's nose to the grindstone or have one's nose to the grindstone or hold one's nose to the grindstone {v. phr.}, {informal}
To work hard all the time; keep busy with boring or tiresome work.
Sarah keeps her nose to the grindstone and saves as much as possible to start her own business.
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Source: A Dictionary of American Idioms
