(as) hard as nails

Physically or mentally tough. I'm scared of those bikers—they all look like they're hard as nails! Our aunt is as hard as nails, so we asked her to give the eulogy, knowing that she's the only one who could do it without crying.

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.

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.

If someone is as hard as nails, they are very unsympathetic towards other people and do not show their emotions. He's a shrewd businessman and hard as nails. When necessary she could be as hard as nails. Note: You can use hard-as-nails before a noun. That was his hard-as-nails trade representative, Carla Hills.

Unyielding, tough, usually describing a person. This simile, which replaced the earlier hard as flint or stone (dating from Chaucer’s time), seems to allude to a nail’s ability to withstand the blows of a hammer. Shaw used it in at least three of his plays (You Never Can Tell, Heartbreak House, Man and Superman) to describe an unsentimental character.