"Tu peux te brosser" or "tu peux toujours rêver" means you can keep dreaming or in your wildest dream. It's a useful expression when you try to tell someone to be more realistic or to give up on what they want to do. The example in the episode: "Puis-je emprunter ton ordinateur pour finir mon devoir?" "Umm.. Tu peux te brosser, mais non!"