I've thought about it and I'm not going to give my friend his money back.
Is there something he did wrong that makes you not want to return his money?
Sorta, but it's mostly because he's a close friend, so it's not too serious.
You are close friends, so money that you borrow from such people should be paid back in a timely fashion. Not paying back your close friends is a breach of their trust in you and may end up destroying the relationship.
I'm not giving him the fucking money back.
The responsible and moral thing to do is pay your friend the money that you owe him. Him losing trust in you will harm your friendship and it is ultimately a form of stealing. How would you feel if the situation was reversed?