It's an inversion of the racist phrase - when one black calls another black man 'n**ger', it's interpereted as friendly banter - equivalent to 'mate', but if a white man says it, even to a black man he knows well, he's on dangerous ground.
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In a duplicate question, SimonC wrote: Because, rather peversely, to even ask the question labels you as a racist in our sad PC society. Listen to any album by Dr Dre, Tupac and the word is used freely between people of the same origin. Interestingly, it seems OK for a hispanic person to refer to a black person as a ******, witness Jennifer Lopez in the remix version of "I'm real".