As a native New Yorker, it pains me to see that none of the other definitions seems to actually grasp what not for nothing means.
"Not for nothing" is used to soften the blow of something that would normally be offensive or come on too strong.
For example, if your friend showed up wearing purple and yellow striped pants to a party and asked you how he looked, you could respond, "well, not for nothing, Jimmy, but you look like a gay, retarded zebra." Or if you want to try out for a musical but you can't sing to save your life, your friend might tell you, "hey, not for nothing, but you suck at singing and you're not gonna get that part." Things of that nature.
Not for nothing, but the other guys who defined "not for nothing" are a bunch of schmucks.