Idiom: I (he/she, etc.) wasn’t born yesterday
Meaning: Something that you say in order to tell someone that a person is not stupid and cannot be easily deceived
Selena: Kelly, this place is a mess! I thought you were going to clean everything up and decorate it for the party tomorrow. That is what you told mom.
Kelly: I did! I’ve been cleaning all day long. I only have a few more things to do. Please back me up on this with mom.
Selena: Seriously? She wasn’t born yesterday. She knows you’ve just been sitting around all day watching the whole season of Game of Thrones again!
Kelly: Well, that show is so addicting. I just couldn’t stop.
Selena: We might as well finish up here before mom gets home.
Jennifer: Wait a second….Tommy, what is this on your shirt? It looks like pink lipstick.
Tommy: Huh? Oh no. Is that lipstick? Um….let me think. When I was on the bus today, a woman with really high heels stumbled while she was walking down the aisle and her face fell right on my shirt.
Jennifer: You know what? I wasn’t born yesterday. I’m surprised you couldn’t come up with a better story than that, cheater!
Meaning: In Context 1, Kelly claims that she cleaned everything up for her mother, but it is obvious that she hasn’t done anything. Selena tells Jennifer that her mom will not believe her. In Context 2, Jennifer finds lipstick on her boyfriend Tommy’s shirt and asks him about it. When he makes up a crazy story, she tells him she doesn’t believe him and is not stupid by saying “I wasn’t born yesterday.”