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.