Originally, it was a somewhat common mispelling of the word "brother", often by baby boomers, people attempting to imitate Hulk Hogan, and Fudds. It's now used often to troll people, and is used as a mild joke when shitting on boomers in
gun communities, and biker communities. It's also especially
popular on /k/, other boards of
4chan, when adding complete silliness to a post.