Adjective
(simile, idiomatic) Very unsympathetic; callous
(simile, idiomatic) Able to endure difficulties; tough
Source: en.wiktionary.orgI admired her dedication and held sympathy for her tough upbringing but she had a hard as nails exterior with no sign of a softer side for a really long time. Source: Internet
It’s hard as nails. Source: Internet