Stands for Real Nigga Shit. A phrase used when someone wants to add emphasis to the fact that the thing they just said was, indeed, true. It is a phrase or sentence someone says that is so true it deserves to have "Real Nigga Shit" at the end of it.
Aye man if you come in my face like that again I'm knocking yo ass out, Real Nigga shit(RNS)!
Retarded Nose Syndrome. If one has a chronically runny nose from a long-lasting illness, or just unlucky enough to have abnormally runny nose genes, they are said to have RNS. Also sometimes known as Runny Nose Syndrome, for those easily offended.
Heather: Why do you have such a huge pile of nasty tissues by your desk?
Me: I have an acute case of RNS, once it starts it takes hours to die down!
Heather: Real nice.