"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!"