So amandab asked if anyone in the IRC if we could research the word "Bro." We had an entry on "Swag" a few months back, so this seems like the best logical step. I'm starting this thread so we can crowdsource stuff before making an entry.
As far as I can tell, the term seems to be directly derived from "brother," and the earliest Urban Dictionary entry I can find is from 2003. It seems to be an invention of the last decade, and it's come into extensive use during the 2000s. We have a few entries with the term, Come at me bro, Cool Story Bro, and a few others.