hard as nails
hard as nails {adj. phr.}, {informal}
1. Not flabby or soft; physically very fit; tough and strong.
After a summer of work in the country, Jack was as hard as nails, without a pound of extra weight.
2. Not gentle or mild; rough; stern.
Johnny works for a boss who is as hard as nails and scolds Johnny roughly whenever he does something wrong.
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Source: A Dictionary of American Idioms
