莫名其妙is an adjective. And it means you cannot understand the things that happened. for example, 你這人真是莫名其妙。 You are so 莫名其妙。(You cannot be understood.You may think that there's something wrong in the person's mind.) 這到底怎麼回事啊，莫名其妙。(what's going on? i cannot figure it out.) 你怎麼莫名奇妙地就和他結婚了呢？ Why you suddenly get married with him.i don't understand.

莫，否定词，此处可以解释为“无法、没有办法”。名，说清楚，解释清楚。其，第三人称代词，或第三人称代词的形容词，他，她，它。。。。。妙，奥妙，玄妙。莫名其妙：无法解释清楚它的奥妙。

莫名其妙的意思是不知道什么原因

Could you give me an example of how I can use this idiom?

It's an idiom: unable to make head or tail of something; baffled