1
general::
Phrase(s): *hard as nails
Cliché [of someone] stern and unyielding. (*Also: as ~.) • Don’t try to bargain with Liz. She’s as hard as nails. • Bob may seem sweet and easily swayed, but in fact he’s hard as nails.
McGrawhill's American Idioms And Phrasal Verbs
2
general::
Unyielding, callous, unsympathetic, as in Don't ask her for a contribution? she's hard as nails. This expression has replaced the 14th-century simile hard as flint stone and presumably alludes to the rigidity of nails.
American Heritage Idioms