The opinion of a bro. Commonly used in a sarcastic sense, and often by girls who are seen as "one of the guys", but also used by other bros.
Girl: I'm never going to get a boyfriend.
Guy: Why not? I think you're awesome.
Girl: What a bropinion.
Guy: Seriously, I do think you're awesome.
Girl: Of course you do, because if you didn't, I would have canned you a long time ago.