bite in the ass不能理解成“咬屁股”，实际上它是一个俚语，表示：自食其果、搬起石头砸自己的脚。

【例句】
If you don't study hard,you'll bite yourself in the ass and fail the exam.
如果你不努力学习，你会自食其果，考试不及格的。

I think it was really a bad decision of them and it’s going to bite them in the ass.
我觉得这是他们一个非常糟糕的决定，而且他们最终将会自食其果。