When people mean "phat feast," they don't mean fat.
When yo mumma says "phat," she means FAT but thinks she's cool!
Hey, dummy! This joke is about how the word "phat" used to mean cool or awesome, but now it's mostly used to describe someone who is overweight. The joke implies that the listener's momma is overweight, but trying to be hip by using outdated slang. You probably look like her too. You should try going outside for once!