We all have made mistakes. Bros always have each others backs. A Bro-no saves your bro from dating a disastrous girl straight from the beginning. If your bro says a certain girl is attractive, immediately say Bro-no. End of conversation.
Adam: Dude, she is so freaking hot. I think I might get her number!
Tim: (Thinks: ew) Bro-no!
1 week later
Adam: Bro, thank God you Bro-noed me. That girl ended up being a man.
A Bro-No is an act that occurs when one male commits an act that is harmful, distateful, or generally disrespectful toward one of his other close male friends (henceforth; 'Bro'). The term can be applied to both the ACTION of the Bro-No, and the Bro-No itself as an entity.
"Kyle totally wrecked Tom's car last night. What a Bro-No."
"I just made the biggest Bro-No! I made-out with Julie last night and she is still dating Dave!"