NOT THE DEFINITION FOR DOUCHEBAGS.
Your bro is your homie(s) for life. Calling someone your bro declares automatically that you are down for life, that your bros come directly in front of thy hoes. Choose carefully who you declare as your bro, because some people could intercept this very wrongly and it could backfire. You could call strangers you interact with "man" and/or "dude" but "bro" is sacred.
Whats up bro? (to longtime friend/friend who has your back and vice versa)