A white guy that goes around talking trash on blacks (including calling them "nigger,")and then will turn around and rap or spout rhymes to impress chicks at a bar. Secretly hates african americans with a passion, but then will behave exactly in the fashion he says he hates them for. The worst kind of hypocrite. "Brett" is used because it's a lily white name and doesn't go with "dogg" well.
"That brettdogg over there is driving me nuts. His rhymes are fuckin' whack."
"I don't like that bar. It's full of brettdoggs."