An adult male whose social life revolves around collegiate homosocial bonding...a young, usually unmarried, often immature guy who just does what everyone else his age seems to be doing.Given what we know about the economy's impact on youth marriage rates, this is just extraordinarily vicious. It is true that young unmarried men are more prevalent than they used to be, but this isn't because of immaturity - it's because they can't afford it. Millennials don't have the luxury their parents and grandparents had of marrying early and building social lives around their families. To fill that void in community, young people have different socialization patterns than previous generations did; this is all an entirely predictable consequence of yawning economic inequality that falls disproportionately on the young.
Especially since the people throwing "bro" around tend to have families - or are well-off enough to afford them - I see no way of uncoupling this slur from one of the more odious symptoms of inequality in America today. It is a way for privileged people to ridicule the victims of their success, and to disclaim responsibility for the economic and cultural conditions they have created by blaming youth marriage rates on immaturity. The subtext of "bro", at least as constructed by Jezebel: bootstrap yourself out of capitalism, get a danged career, start a family like your mother and I did, and stop spending so much time with your friends.