interjection
An interjection expressing surprise, disbelief, or a request for clarification. Often used in casual conversation.
何を！そんなことありえないよ。
What! That can't be true.
何を、今さら謝られても困るよ。
What? Apologizing now doesn't help.
adverb
Adverbial use asking for a reason or cause. Often used in rhetorical questions or expressions of puzzlement.
何をそんなに慌てているの？
Why are you in such a hurry?
何を根拠にそう言うのか。
On what grounds do you say that?