RichThesaurus
hard as nails
vs
stonehearted
hard as nails
adj
Very unsympathetic; callous
Able to endure difficulties; tough
stonehearted
adj
Hardhearted; pitiless; unfeeling.
How often have the words
hard as nails
and
stonehearted
occurred in a corpus of books?
(source:
Google Ngram Viewer
)