I second the “Prdko” idea. It means brave in Bulgarian too.
Btw, real nicknames, unlike usernames or handles are given to you by other people and hence sometimes unflattering. It’s bad form to refer to someone as “козела” or “крушата” when you’re physically weaker or younger, especially when you’re within earshot of the aforementioned. That makes it that much more pleasurable to do it when it’s the other way around. I did know a guy who adopted the moniker “пандиза” purely for street cred, though. I should probably stop giving further examples or the law will be able to triangulate me.