“pain in the ass”别理解成“屁股疼”，其正确的含义是极其讨厌的人（或事物）。

【例句】
That guy is a real pain in the ass. Just stay away from him.
那个家伙真是个讨厌的人。离他远一点儿。

I have to go shopping with my girl friend because she wants to. It's a real pain in the ass.
我必须和我的女朋友去购物，因为她想去。真讨厌啊。