Why shouldn't you get in a fight with a dinosaur?
You'll get jur ass kicked.
Listen up, buttercup. This joke is funny because it uses a pun. "Jur" sounds like "your," but it's also short for "Jurassic," like in "Jurassic Park" with all the dinosaurs. So, instead of saying "you'll get your ass kicked," it says "you'll get jur ass kicked" which is what would happen if you mess with a dinosaur, you idiot.