What it means
Someone hard as nails is tough, fearless and utterly unsentimental, with a shell that nothing seems to dent. Nails are unbending and unfeeling, so the comparison fits the person who shrugs off pain, pressure and hard knocks without flinching. It works for the brawler nobody dares cross and for the steely sort who simply never lets anything get to them.
Usage examples
"You will not see him cry, that lad is hard as nails."
"Gran is hard as nails, she carried on gardening the day after her operation."
"My aunt in Dundee is hard as nails, raised four children alone after her husband left in nineteen seventy-eight, worked nightshifts at the factory until she retired at sixty-five, and still walks the dog three miles every morning before the rest of the family is even awake."
"The new manager at the warehouse is hard as nails, fired three lazy supervisors in her first week on the job, restructured the entire operation in a month and a half, and the team productivity numbers have not stopped climbing since her first Monday morning briefing."
Where it comes from
Sixteenth-century English simile that compared the unbreakable solidity of forged iron nails to the unfeeling toughness of a person. The phrase appears in print from the Elizabethan period onward, and has survived four centuries of language change without losing any of its directness, useful equally for the boxer, the grandmother and the cat with battle scars from the alley behind the chippy.
Other ways to say it
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