In English：someone or something that bothers you very much, but it's hard or inappropriate to put them on the table to talk about because of the bother-maker's identity. 某人或者某件事情使得你很烦恼，但是又不能放在台面上说。

　　来源：

　　When your ass pains, it’s difficult for you to sit and sleep. 试想一下，当一个人的屁股疼的时候，他就很难去坐下来或者入睡。it will always bother you when the pain disappears. 直到痛消失后才不会再烦恼。Sometimes you can say ”pain the butt ” instead of “pain in the ass”

　　举例：

　　For a short guy, sparring or fighting against someone with a height and reaching advantage can be a real pain in the ass.

　　对于一个小个子来讲，和比自己长的高的人打架并且占优势真的是很难的事情；

　　Getting all this spam email is such a pain in the ass. I wish they wouldn't send the damn stuff.

　　来处理这些垃圾邮件真的很烦，我希望他们不要再给我发这个烂东西了。