From Longman Dictionary of Contemporary Englishwhat’s the big idea?what’s the big idea?spokenASK A QUESTION used when someone has done something annoying, especially when you want them to explain why they did it Hey, what’s the big idea? Who said you could use my computer? → big
Examples from the Corpus
what’s the big idea?• Hey, what's the big idea? Who said you could borrow my car?what’s the big idea?what’s the big idea?spoken used when you cannot understand why someone has done something → idea 