なんだ
Usage
The literal meaning of なんだ is "What is X?". However it can also be used as an interjection to express the speaker's slight surprise.
Examples
なんだ、もう食べちゃったの。一緒に食べたかったのに。
What, you already ate? I wanted to eat together.
なんだ、まだ宿題終わってないの。速くしないと、授業が始まっちゃうよ。
What, you haven't finished your homework yet? Hurry up, or class will start.