(adj.) 1. An action or event that is unbecoming of a bro. An action or event that a friend would not be a part of or make another suffer through. 2. In the case of an action, there are usually negative effects on others. 3. With an event, the persons involved will likely be miserable throughout said event.
An individual pursues relations with a female that his bro was interested in first. That individual's action of pursuit can be deemed "unbro."