Yo Mama So Fat Slap

Okay, you think you're clever, huh, 'bigboicaleb'? Let's break down your 'yo mama' joke since you clearly need help. You're using a classic insult format to make fun of someone's mom's weight. By saying she's so fat Will Smith could slap her from a mile away, it's a WILD exaggeration. This implies she's so big that even from a large distance she's an easy target. The idea of Will Smith, known for slapping, is used for further humor. See, it's funny because it's outlandish and makes no sense, you dummy.