Wiktionary
tough as nails
a. (context idiomatic simile English) Having a hard, strong, and determined mindset / mentality.
Usage examples of "tough as nails".
He was ready to take the case to court, and what chance would I have had against theman unmarried career woman, tough as nails, probably 'queer' that's the word he would have usedagainst a pair of doting, God-fearing, cookie-baking, ideal grandparents?