It is a noun or adjective that describes a very close friend of yours that is almostlike a brother to you.
Combination of the words negro and bro. By replacing the "gro" with "bro" you remove all doubts about the racism so you can say the word without the fear of offending someone and in the end feeling betterabout yourself.
When a bro knows that a girl has a disease and doesn't run interference for another bro who is trying to get with her. This is usually done to spite another bro very few incidents of this are accidental.
Todd: Hey man don't you think we should warn Garret about her?
Mike: Nah, I think a little unbrotected sex is just perfect for him.