as hard as nails 冷酷无情的；铁石心肠的；强硬好斗的

【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。

They are merciless, with hearts as hard as nails.
他们冷酷无情，铁石心肠。