" All over America, infuriating white people would address black men with the words “Hey, boy.” And it grated. It really grated. That’s why, in the 1940s, black Americans started taking the fight the other way and greeting each other with the words “Hey, man.” The vocative was not inserted for the purposes of sexual identification; it was a reaction against all those years of being called boy. It worked. White people were so confused by “Hey, man” that the sixties happened and everybody, of whatever race, started calling each other man, until the original significance was lost. This is an example of Progress. Now, "

― Mark Forsyth , The Etymologicon: A Circular Stroll through the Hidden Connections of the English Language