An American idiom used sarcastically to mean that something’s fake. When something’s genuine, that means it’s real. But the U.S. never made three-dollar bills, which means that there’s no such thing as a genuine three-dollar bill. For example, the Louvre Museum in Paris has the genuine Mona Lisa. If your school has a picture of the Mona Lisa hanging in the art room, it’s not genuine. It’s a copy. That means your school’s copy of the Mona Lisa is as genuine as a three-dollar bill.