何なんだと

expression, interjection

What did you say?!; What's that?!

A confrontational or surprised retort, often used when the speaker is offended or incredulous about what was just said. Can be rhetorical or a genuine demand for clarification, depending on tone.

• 「お前まえ、役やく立たたずだな。」「何なんだと！」

  "You're useless." "What did you say?!"

• 「明日あしたから会かい社しゃに来こなくていいよ。」「何なんだと？」

  "You don't need to come to work from tomorrow." "What's that?"