interjection
what?!; what did you say?!
An exclamation of surprise or disbelief, often used when hearing unexpected news. Casual and conversational.
え、なんだって？もう一度言って。
Huh, what?! Say that again.
彼が辞めるって？なんだって！
He's quitting? What?!
adverb
why?; what for?
Used to ask the reason for something, often with a tone of surprise or demand. More emphatic than なぜ.
なんだってそんなことしたの？
Why on earth did you do that?
なんだって今頃来たんだ？
What are you doing here at this hour?
adverb
Used in the pattern なんだって～ to mean 'anyone' or 'anything', similar to 何でも. Often followed by a verb like できる or いい.
See also: 何でも
彼はなんだってできる。
He can do anything.
なんだっていいよ。
Anything is fine.