What's the big idea?

Meaning

A confrontational way to ask why someone is doing something annoying or disruptive. It implies that the action is foolish and the person should have known better.

Confrontational, classic

Used in a sentence

• He dumped water on my head as a prank and I asked, what's the big idea?
• When my brother took my keys without asking, I shouted, what's the big idea?
• The security guard stopped the kids and asked, what's the big idea messing with that fence?