From Longman Dictionary of Contemporary English(as) hard as nails(as) hard as nailssomeone who is hard as nails seems to have no feelings such as fear or sympathy → hard
Examples from the Corpus
(as) hard as nails• Willie O'Connor is as hard as nails and Liam Simpson takes no prisoners.• Hard as nails he may be but don't let anyone tell you Hank hasn't got a sense of humour.• Beautiful but hard as nails, she'd thought then. 