An interjection expressing surprise, indignation, or disbelief (as in 'What!' or 'What are you talking about!').
何を！そんなこと、信じられない。
What! I can't believe that.
A: 彼は全部自分でやったんだよ。 B: 何を言っているんだ、それは無理だよ。
A: He did it all by himself. B: What are you saying? That's impossible.
何を！私たちが諦めるわけがない。
What! There's no way we're giving up.
Used to ask 'for what' (purpose, reason, or thing). Functions as a question word meaning 'for what' or 'why'.
あなたは何を目的にこのプロジェクトに参加したのですか？
What is your purpose for joining this project?
何を今更そんなことを言うんだ。もう決まったことだよ。
Why say that now? It's already decided.
何を頼みたいのか、まずははっきり伝えてください。
Please first make clear what you want to ask for.