MUSIC: Sky Sailing - Tennis Elbow

He's a hard nut to crack. 

他是一个难对付的家伙。

“hard nut to crack” 先理解一下表面意思：

hard 硬的，在这里用来强调 nut 很硬；

nut 坚果，表示坚果有硬壳很坚硬，不易砸开

crack 裂缝，声变，破裂，变声

As we all know, Our boss is a hard nut to crack, every time we want to ask for a leave, he simply says no, whatever reason you might have.

A: Why do you look so upset?

B: Don’t you know that we are going to

have a math exam tomorrow?

A: So what? Relax. It is a piece of cake.

B: It might be a piece of cake for you, but for me, it's a hard nut to crack.