(US,colloquial, often preceded by hey)Used to express surprise or dissatisfaction with an action or statement of another person, especially to that person.
I heard you been chatting up my girl. What's the big idea?
Hey, what's the big idea, you telling everybody I'd left town?
Although the question can be read as "What is the purpose ('big idea') of what you just said or did?", it is almost exclusively used to object to whatever was just said or done by the hearer.
