Fat person: "Hey, what's up?"
Friend: "Your blood pressure!"
Fat person: "Hey, what's up?"
Friend: "Your blood pressure!"
Listen up, buttercup! The joke is that the "friend" isn't just asking "what's up" as in "what's going on." They're implying the fat person's blood pressure is literally "up" because they're overweight. Get it? You probably don't. You are dumb.