brother

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

Example:
"Hey, did you take out the meat from the freezer?"
"Uh, no... I forgot."
"Brotherrrrrrrrr"