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.
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The American Heritage® Idioms Dictionary Copyright © 2002, 2001, 1995 by Houghton Mifflin Harcourt Publishing Company. Published by Houghton Mifflin Harcourt Publishing Company.