as hard as nails 铁石心肠；冷酷无情；毫无惧色

【例句】
You'll never move him to kindness; he's as hard as nails.
你决不会使他发善心，他可是铁石心肠。

He won't do you any harm. My stepmother might. She's ignorant, but she's as hard as nails.
他一点也不会伤害你，但我的继母却可能会害你。她愚昧无知，但铁石心肠，而且很狡诈。