I’m not sure who started it or when. But Black men refer to each as “Brother” all the time.
Even black cops on The First 48 will often say to suspect “My Brother” and ask a question. Or you’ll hear witnesses say “That Brother is ***** crazy”.
I have a couple ideas. That it came out of the Black Power Movement in the late 60’s and early 70’s. The raised fists. But that’s pure speculation.
It’s common throughout the Baptist churches (probably other denominations too) to call someone Brother Name. for example Brother Jackson or Brother Jones. But they never say “That Brother” or just walk up and address the person as My Brother. They’d say “Brother Jackson, how are you today?” That’s how it’s done in every Baptist Church that I attended. Dr King of course was a Baptist Minister.
What’s the Straight Dope?