A coon is someone who is typically a sell-out, or is a representative for the African-American stereotype, so a coon ass nigga is a coon to the eXtreme
50 cent is hella coon ass nigga.
"Damn tyrone, G-Unit is a buncha coon ass niggas"
"Damn that girl tiquanda datin that coon ass nigga, can't even see his ass in the dark."
"Damn blubaugh one hella wigger coon ass nigga wanna be"