Referring to a fat girl a guy has slept with or would sleep with but denies it venomously, especially to his friends. He may even ignore her completely unless she is naked on top of his dick. Origin: "Why’s a fat girl like a moped? They're a lot of fun to ride, but you wouldn't want your friends to see you on one."
"I think Mike has been riding a Moped."
"She has the hots for you, she might be fat and ugly but she's still a Moped"