When did musicians started to call each other motherfucker in a friendly meaning?

Question

According to the Cambridge Dictionary, the word motherfucker is a very offensive way to insult someone.

an extremely insulting name for someone you hate or for someone who has made you angry.

But I can hear very often musicians using this term, in lyrics, live shows or interviews, to call their friends or to use it as a mark of respect to someone.

For example, in his song Agains All Odds, Tupac says:

Hopin' my true motherfuckers know
This be the realest shit I ever wrote

This means:

I'm hoping my true friends know
This is the realest words I ever wrote

By the way, we can also note he's using the word shit to qualify his own work, but I guess not in a negative meaning.

So this is an unexpected reverse of meaning. I wonder how it started in music.

Answer 1

This isn't, strictly speaking, a musical phenomenon, although it is most familiar to many people through music. Playful, affectionate, casual and creative (and often counterintuitive) use of profanity is a common marker of the speech of people in lower socioeconomic classes around the world.

In America it is particularly associated with African-American culture (both legitimately, and through exaggeration in the media). Because of the central importance of African Americans to American popular music, and the music of the modern world, many characteristic speech patterns, including this one, have thus been popularized and disseminated.

This is particularly noticeable in the world of hip-hop (and prior to that, the blues). Whereas the influential Motown sound of the 60s was a black musical form that had deliberately been shaped to appeal to a crossover white and middle-class audience, hip-hop is predicated around a concept of lower-socioeconomic "authenticity," which encourages even its more middle-class practitioners to affect the "rougher" idiom of the streets.

Answer 2

An early and registered occurrence of this world was in 1981. Miles Davis complimenting the percussionist Mino Cinelu.

The book The Last Miles: The Music of Miles Davis, 1980-1991 (p90) says:

In 1981 Cinelu was playing in an R&B combo called The Frank and Cindy Jordan Band. ( . . . ) They were dur to play at Mikell's, Cinelu asked if he could switch to percussion: "I wanted to get back to playing percussion, so it was just by luck that when Miles waw me he wanted a percussionist," says Cinelu.
( . . . ) The night Cinelu played, Miles was sitting in the front row of the audience. "I didn't recognize him. At the time his health was not great," recalls Cinelu. ( . . . ) When the set was over, Cinelu stepped off the stage and walked past Miles who grabbed his arm and said "You're a motherfucker." Cinelu thanked Miles for the compliment and then pulled Mile's hand off his arm before walking away. Then Mikell said to Cinelu: "Do you know who that was?" When Cinelu shook his head, Mikell told him the identity of the mystery admirer. "I said 'Oh shit' and I went back and told Miles it was a pleasure to meet him, but Miles ignored the small talk and asked me for my number."

An interview of Mino Cinelu in Pop Matters says:

There was a guy in the front row looking really really intense. He was listening to the music like no one else in that club. The set finished. This guy grabs my arm; he’s really strong. He looked older. You could not recognize him, especially because he was not in the best of health. He said, “You’re a bad motherfucker. Give me your number.” I tried to move away, not really to defend myself but I was sensing the situation. I said, “My name is Mino".