沪友详解A：Joe叔叔很是冷酷无情；尽管他是个百万富翁却不会去帮助他的穷亲戚。
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。如果一个人像钉子一样硬的话，那人想必既顽固强硬又无情无义，是个相当难相处的人。

沪友详解B：as hard as nail 表面意思是象钉子一样硬,根据句意我个人认为可以理解为吝啬或倔强.

as hard as nails
英汉：adv. 身体结实, 冷酷
例句：
1、She 's hard as nails.
她身体很结实。
2、The soldier was as hard as nails.
这位战士身体如铁打一般的强健。
3、After a summer of work in the country, I was as hard as nails, hard as nails, without a pound of extra weight.
在乡长下劳动一个暑假之后，我的身体十分强壮而体重并没有增加。
4、They are merciless, with hearts as hard as nails.
他们冷酷无情，铁石心肠。