African-American jazz musicians in
the 1930s began calling each other "man" as a
sarcastic comment on,
and way of standing up to, the widespread
racist use of the word
"
boy" to refer to adult
black men.
According to Ken Burns' documentary Jazz, this is the origin of our contemporary usage of "man" as an informal all-purpose
vocative (e.g., "Hey, man, how are you?", or "Did you hear about the E2 IPO, man?"). The OED quotes a source from 1960,
though nothing from the 1930s, describing that as the intent of the usage.