风马牛不相及 fēng mǎ niú bù xiāng jí

　　Have nothing to do with each other. In ancient times, there is such a long distance between Qi and Chu that horses and cattle wouldn’t enter the other side.

　　【Example1】这和我们的谈话内容，有点风马牛不相及啊!

　　I don't see it has anything to do with what we're talking about.

　　【Example2】在音乐课上，一个数学的问题是风马牛不相及的。

　　A question about arithmetic is irrelevant in a music lesson.