a casual way to ask 'what' (colloquial form of 何)
なあに、そんなに驚かなくてもいいよ。説明するから。
What is it? You don't have to be so surprised. I'll explain.
なあに、それはただの冗談だよ。
What? That was just a joke.
子どもが指差して『なあに？』と聞いた。
The child pointed and asked, 'What's that?'
an interjection expressing surprise, roughly 'what?!' or 'oh?'
なあに！そんな大変なことが起きたの？
What! Something that serious happened?
『宝くじに当たったよ』と聞いて、なあにと目を丸くした。
Hearing 'I won the lottery,' she exclaimed in surprise, 'What?!' and widened her eyes.
an interjection expressing irritation or anger, similar to 'hey!' or 'what the—!'
なあに、そんなこと言わないでよ！
Hey! Don't say that!
また遅刻か。なあに、いい加減にしてくれ。
Late again? Hey, cut it out.