A "McNigga" is a black man that has lost all of his street cred and negro like qualities (i.e. pronouncing it "mouth" instead of "mouff" and using "I don't know anything" instead of "Nigga I ain't know nothin"). This "unblackness", if you will, happens most frequently when black people find a steady, well paying, job and gets over their crack addiction (easing him/herself onto cocaine instead). Luckily, this doesn't happen very much.
"McNigga" can also be used as an exclamation/gibberish when one is excited or can't think of something better to say.
Ross(Ben): that nigga is watchin' seinfeld
Al taymalikshawn(Mike): McNigga!
Ross(Ben): you goin' to the physics review session?
Al taymalikshawn: McNigga!
Ross(Ben): I'm gon stab youse in the dick nigga!