An opinion shared between two 'bros'
Friendly advice that could only be dished out by another bro
In Mike's bropinion, Nate should have dumped that bitch before she got pregnant
When one bro asks for advice from another bro, he has asked for his bropinion.
Hey bro, I need your bropinion on this chick I'm seeing
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.