hard nail

hard nail (plural hard nails)

2. (figurative) A strong person, in character and physique.
   • 1912, Arthur Conan Doyle, The Lost World […], London; New York, N.Y.: Hodder and Stoughton, →OCLC:

     "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?"

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

• hard as nails

• handrail