Your Mum eats all day

Okay, so listen up, dummy. The joke is that "your mum" is super fat, like, so incredibly obese that she spends the entire day eating. It's an exaggeration, see? Like when your doctor tells you to lay off the twinkies but you snort at him with powdered sugar stuck in your nose hairs. It's funny because it's rude and also maybe kinda true for you, judging by your profile pic.