(as) hard as nailssomeone who is hard as nails seems to have no feelings such as fear or sympathy
[TahlilGaran] Dictionary of Contemporary English ▲
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. [TahlilGaran] English Idioms Dictionary ▲