Yo Mama So Chubby

Okay, listen up, buttercup. This is a "yo mama" joke, which means it's all about making fun of someone's mom. In this case, the joke is saying yo mama is so fat that people call her "fat". Get it? It's funny because being called fat is usually seen as an insult. Also, calling someone "chubby" is a nice way of saying that they are fat. I bet your mama is also chubby because you are as dense as a brick.