Pain: P-A-I-N,名词，痛苦，苦恼；ass： 屁股；
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。
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”
For a short guy, sparring or fighting against someone with a height and reaching advantage can be a real pain in the ass.
Getting all this spam email is such a pain in the ass. I wish they wouldn't send the damn stuff.