A : Andy refused all his classmates' invitation.
安迪拒绝了班上所有人的邀请。

B : I really couldn't understand him.
我这不明白他这个人。

A : He is surely a hard nut, nobody likes approaching him.
他真是个脑子不开窍的人，没人愿意接近他。

B : We'd better leave him alone.
我们最好也别惹他。

a hard nut
nut除了指“坚果”的意思，还可以指“难题、难事、难对付的人”。
A hard nut源于短语a hard nut to crack或者是a tough nut to crack，表示“顽固不化的人”“无知无觉的人”“顽冥不化的人”。

leave alone：不理睬，不干涉，置之不理，不打搅

He will leave alone things that might cause trouble; as he would say,"let sleeping dogs lie." 
他从来不干招惹是非的事。他会说：“莫惹是非”。 

You wouldn't see her leave here all alone? 
你不会看她一个人孤单单地离去吧。 

Leave my books alone. 
别动我的书。 

Just buzz off and leave me alone! 
走开, 让我清静一下!