Like the infamous "that's a dip" it means that a person who is being spoken to has made a mistake of something really obvious
after it is said the person who made the mistake is usually hit on the forehead

The guy who no one really knows . He's nice to everyone and has green eyes you want to kiss. He has an obsession with politics but will be nice to you no matter your view.You will fall in love with him but he wont fall for you