be not born yesterday

To not be naïve or inexperienced; to be knowledgeable, intelligent, or shrewd. Don't think you can fool me with that old ruse—I wasn't born yesterday, you know. I know you think Mom is clueless about these things, but she wasn't born yesterday. A: "Why did you send a kid to be my partner on this case?" B: "He may be young, but he wasn't born yesterday—he got his start working homicide in New York City."

I wasn't born yesterday

I am not extremely naïve, gullible, or unintelligent. When one is referred to as "born yesterday," they are being likened to a newborn baby, who has no experience with the world. Don't think you can fool me with that old ruse. I wasn't born yesterday, you know. Of course I know that major political issues can't be fixed overnight. 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!

I wasn't born yesterday

used to indicate that you are not foolish or gullible.

I wasn’t born ˈyesterday

(spoken) used to say that you are not stupid enough to believe what somebody is telling you: You don’t expect me to believe that, do you? I wasn’t born yesterday, you know.