Acronym for "I'm just sayin', nigga."
This allows white people to say the word 'nigga' (note: there is no 'er') in public and not be crucified and/or beaten to death because of it.
Origin: watching one too many Katt Williams and Paul Mooney stand up bits as a white person.
It was not created with the intent to be used maliciously, but to be used as an affirmation for whatever full-of-win statement you just made that preceded it.
Scene: Two friends in a club and a couple of girls walk by...
Person 1 (who's black): Hmm... I don't know. She looks a little crazy.
Person 2: I bet she's a freak. I'm not saying I would (lay her), but I'm not saying I wouldn't. IJSN.