what's the big idea?
used when someone has done something annoying, especially when you want them to explain why they did it

[TahlilGaran] Dictionary of Contemporary English ▲

What's the big idea?
[informal]
something that you say when you want to know why someone has done something that annoys you.
What's the big idea? That's my lunch you're eating.

[TahlilGaran] English Idioms Dictionary ▲

What's the big idea?
Why did you do that?, What are you doing?
"What's the big idea? Why are you using my bicycle?"

[TahlilGaran] English Idioms Dictionary ▲

what's the big idea
informal What is the purpose; what do you have in mind; why did you do that; what are you doing; how dare you.
■ Often used to question someone or something that is not welcome.
The Smith family painted their house red, white, and blue. What's the big idea?
What's the idea of coming in here after I told you not to?
I heard you are spreading false rumors about me, what's the big idea?

[TahlilGaran] English Idioms Dictionary ▲