as hard as nails解释"像钉子一样硬"。我们知道钉子是用铁或者其它非常坚硬的材料做成的，引申为冷酷无情的；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。

There's no point in asking Harry for a donation for starving children; he's as hard as nails.
不用要求哈里给饥饿的孩子们捐赠些东西，他是个冷酷无情的家伙。