1. Abbreviated phrase of the longer expression "I'm not sayin, Im just sayin," meaning to merely suggest an idea or comment without making it your personal opinion or belief.
2. To say something aloud without advocating or supporting it
"Dude why do poor people have so many fucking children? They should have to agree to permanent birth control before they apply for welfare."
-- "Hahah so you think poor people shouldnt have kids???"
--" I dunno Im not sayin, i'm just sayin