hard nail

plural hard nails

1. figurativelyA strong person, in character and physique.

   "He's a hard nail, is Jack, and a dead shot, too, but you can't leave a Grand National winner to die like that - what?"

   1912, Arthur Conan Doyle, The Lost World […], London; New York, N.Y.: Hodder and Stoughton, →OCLC

2. Used other than figuratively or idiomatically: see hard, nail: for example, a steel nail that is hardened.