A rather old slur directed at Irish people. In the late 1800s, the Irish were viewed as a secondary race by White Supremacists (see "Inbreds, Elite English") as were black people. (Who were conversely referred to as "Smoked Irish." I shit you not. People back then had ISSUES.)
These days, the terms has been embraced by Irish wannabe gangsters and thugs, et al, as a badge of honour. Look to House of Pain and Eminem as heroes.
Chap one: "Cracka mutherfucker! Get yo ass out of here!"
Chap two: "Sorry, my bad!"
Chap one: "Yo, you talk funny. Where you from?"
Chap two: "I'm Irish."
Chap one: "Sheeeet, you shoulda told me you was a nigga turned inside out! Gimme some love!"
A real nigga doesn't brag about being real as long as he knows it. His future doesn't seem hopeless. A real nigga stays out of jail, handles shit, and he keeps focused.
That guy who doesn't know what he's talking about but tries to sound like he does. Not very bright, doesn't make any sense (though he thinks he's really profound, on to something, etc.). He's not lying to you, he simply doesn't know any better, but will insist he's right, which makes him not worth the trouble of setting straight. Yes, THAT guy.
A very on-point term (coined by comedian Katt Williams in The Pimp Chronicles pt 1), because we all know one...or more.
Examples of smart dumb niggas would be:
-the chick who insists her Infiniti Q45 is front wheel drive when it's really rear drive
-the guy who spells the word "ludicrous" like the name of the rapper Ludacris
-most people who say overstand or conversate
-one who knows the Bible like the palm of his hand but argues the Earth is only 6000 years old -most conspiracy theorists
You know a smart dumb nigga when you see one basically.