上から
なんでそんなこと聞くの？
なんでそんなことを聞くのかわからない。
単純に
Why?と聞き返しても相手に質問の意図がわからなかったと通じると思います。
こういう言い方も良いと思います。
Sorry, I don't get it. What do you mean?
ごめんよくわからなかった。どういう意味？
What do you mean by that?
それってどういう意味？(何が言いたいの？)
関連
I have nothing to do with it.
それ僕には関係のないことだよ。
It's none of my business
そんなこと知ったことか。
It's none of your business.
君には関係ないだろ。
If this is said in the right tone it will not sound rude but concerned. Therefore it is important that you ask in a polite manner because you do not want to sound rude.
A very common, simple way of asking this question is, "why do you ask."
This can be used is various settings, both formal and informal.
You can also say, "why do you ask me that."
A: Do you have any plans tonight?
B: No. Why do you ask?
C: Do you have any children?
D: Why do you ask me that?
C: Because tomorrow is "bring your children to work" day.
"why do you ask." (どうして聞くの？) がこういった状況では、最も一般的に使われる表現です。
フォーマルでもインフォーマルでもどちらでも使えます。
また、 "why do you ask me that." (どうしてそんなこと聞くの？) と聞いても構いません。
A: Do you have any plans tonight?
→今晩予定ある？
B: No. Why do you ask?
→どうして聞くの？
C: Do you have any children?
→子供いる？
D: Why do you ask me that?
→どうしてそんなこと聞くの？
C: Because tomorrow is "bring your children to work" day.
→だって明日は「子どもを 職場に連れて行く日」だからね。
1. Here, you are interested to know the reason for someone asking you a particular question. Why should it be directed at you? And why would you be asked that question? Maybe it is an inappropriate or unusual question..or there is no reason for asking that question.
2. You are bewildered why someone asks this question. Maybe the questioner is drunk?
Example
"can i ask why you ask".
or
"why do you need this information".
or
"i'm not sure why your asking".
例文
"Can I ask why you ask".
あなたが質問する理由を聞いても良いですか？
"Why do you need this information".
なぜこの情報が必要なのですか・
"I'm not sure why your asking".
なぜあなたが尋ねているのか分かりません。
"I don't really understand why you are asking me that"
If somebody is asking you something and you do not really understand why, you could express this by saying any of the following: "Why are you asking me that? I do not understand", "Why do you ask?" or "I don't really understand why you are asking me that".
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