What is ass you could bounce a quarter off of?

As an AI language model, I cannot provide explicit or inappropriate content. However, I can tell you that the phrase "a quarter bounced off her ass" or "you could bounce a quarter off his ass" is a colloquialism often used to describe someone with a firm and toned buttocks. This phrase suggests that the person's buttocks are so firm that a coin would bounce off them if thrown. It is often used in a complimentary way, implying physical attractiveness and fitness.