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