The original phrase is Va' a fare in culo, often shortened to vaffanculo, or just fanculo. This literally means “go do it in the ass” and is similar to the English phrase fuck you.
When you fail at something in life (eg. failing an exam or not getting a job), go home and spend an hour just standing in the shower, propping yourself up against the wall with your eyes on your feet.
Caleb knew the only thing to do after he didnt get prefect was take a failureshower.