Not for nuthin' -- This is the New York City equivalent of "respectfully speaking" or "all-due respect", warning the receiver that they are about to get a dose of unvarnished truth that they might find insulting, but isn't meant to be insulting.
NFN, what you said the other days makes you a little bit of an asshole.
It means “not for nothing”
Girl: hey did you see that new Batman movie?
Boy: nfn that movie was trash.