This term originated in theatre, coined by the venerable theatre critic Allison Ward. It can refer to a variety of situations involving a bro, but the following are the most common uses of it:
1)A friend who is not really your friend, thus for show
2) a bro in your show, thus a "show bro"
3) a bro who aids your show, whether by working in the crew, or driving you around to obscure prop shops to pick up swords.
4) a bro of whom you are proud, and thus flaunt continually
1) "I though that Karen was my friend, but she just told my secret to everyone!"
A hoard of bros, who not only stroll down the halls together screaming obnoxious obscenities, but also monopolize the showers with their communal chants and brotherly affection.
I couldn't take a dumpwhen the bro showers were happening because their incessant shouting made my shit scared.
12 step recovery jargon referring to someone new who talks about how great life is, now that they'resober. Usually meaning that the person is out of touch with reality.
The new guy seems pretty happy for a dude who has no job, no money and no family. He must be on a pink cloud.