Around both Jakarta and Jogja on this recent trip, I saw the slang-y phrase “Mas Bro” everywhere. I’d seen it before, but it seems much more common this trip than before.
“Mas Bro” is a uniquely Indonesian construction. It combines “bro” (as in, well, bro, brostep, etc.) with the Javanese-inspired term of address “Mas,” used to refer to a young man. So it’s redundantly redundant. I can’t quite tell, as I’m not fluent enough in bahasa gaul, but I think using it even has the bro-y connotation that bro (or “brah”) have.
Ada apa mas bro? = S’up brah?
And, because Indonesian slang is endlessly playful, the way that you address a young woman is to replace the Mas with Mbak, its female equivalent. So, Mbak Bro.