be born yesterday

To be naïve or easily deceived. Often used in the phrase "I wasn't born yesterday." Oh, you expect me to believe that you've been in your room all night and didn't just sneak in the house a few minutes ago? Well, I wasn't born yesterday—and I heard the door open! Do you think I was born yesterday? There's no way that email isn't a scam! Don't think you can fool me with that old ruse, I wasn't born yesterday, you know.