A type of sibling that sucks to deal with. They scream loudly and always ask you to do their schoolwork. Younger or older they suck. If you don't have a brother you are a luck master.
Instead of brother being a formal term for a male friend, brother here is used as an expression. It expresses disappointment, surprise, and much more. Replaces how "bruh" is used. Similar to how "oh brother" is used.
In short,
brother is long for bro;
bro = bru
brother = bruh