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.