hard as nails

hard as nails idiom
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.

The American Heritage® Dictionary of Idioms by Christine Ammer. Copyright © 2003, 1997 by The Christine Ammer 1992 Trust. Published by Houghton Mifflin Harcourt Publishing Company. All rights reserved.

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