Your Mum is Fat

Okay, listen up, dummy! This joke is all about how your mom is super, duper fat. Like, so fat that when she plops her caboose down, she's all, "Whoa, why are there so many people under me?" Get it? Because she's so big, she's basically sitting on a crowd of peeps! You probably don't get out much, which is why you don't understand simple jokes.