Bro and dude
I'm pretty sure "bro" is a southern term, short for "brother", which is short for "Brother in Christ", a traditional greeting in the Baptist and other churches.
Black Americans brought this usage north, and still routinely use "brother" and "sister" to mean an African American man or woman.
"Bro" is no longer current among hip young black folks, but it's still used by a lot of non-black Americans.
Living in Los Angeles, I hear "dude" used a lot more than "bro". In the early 20th century a "dude" was an affected overdressed male, like a dandy or a fop. (Okay, Xbiker, this is your chance to post: That's so GAAAAYYYYY!") A "dude ranch" was a place where guys liked to dress up like cowboys, but didn't do real cowboy work.
Somehow "dude" changed from meaning sort-of-a-sissy to meaning any male friend or buddy.
(I can't help myself; I used to be an English teacher!)