hard as nails

1. 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.

Related Words

• decisive 
• dogged 
• purposeful 
• resolute 
• resolved 
• serious 
• single-minded 
• steadfast 
• strong-willed 
• stubborn 
• tenacious