“What? That's nuts!”
Your friend is telling you about the traffic jam that made her two hours late to work. You show your surprise.
What? That's nuts!
You say "What?" when you can't hear or understand what someone says. But you can also say "What?" when they say something surprising:
A: I've decided that I'm dropping out of school.
B: What? Are you serious?
"Nuts" is a casual way to say "crazy". As an example of how to use "nuts", if your friend is going to do something risky you might say:
When a friend or coworker says something that's surprising, you can respond, "That's nuts."
A: We hiked 45 miles in two days.
B: Whoa, that's nuts! Really?