What does "not for nothing" mean?

"Not for nothing" means, literally, "for something," but should be interpreted as "there is a good reason that..." It is used to describe something fitting.

In its straightforward "literal" sense, not for nothing is equivalent to for a very good reason.

But lately in the US it's used idiomatically (often followed by but) defined by UrbanDictionary as...

used to soften the blow of something that would normally be offensive or come on too strong.

There's also this Wordpress post giving the meaning “What I’m about to say is important” - which "sorta" makes sense as a "literal" usage (I have a good reason for saying what I'm about to say), though I must admit I've never come across it. But it can be seen as a "bridge" between the literal sense and the otherwise impenetrable idiomatic usage above (I have to tell you this because it's important that you should know it, but I don't want to tell you, because you won't like hearing it).

In Dutch there is the saying "Niet voor niets", it means literally the same as the English version. Could it be the case that Dutch native tongues introduced the phrase by mistake?

I have always interpreted hearing this as "Naught for nothing," rather than "Not for nothing."

As mentioned above, I can tell you that is it not for nothing that I am typing this response. But the way that I hear people use this phrase is always followed by a "but" and then some opinion or advice.

So I heard it as "naught," and assumed that it meant "I am offering this opinion, and its value is nothing but you have paid nothing for it." Or, "here is nothing in exchange for nothing." You may also just as easily be saying "I know you didn't ask my opinion, but..." or "What I am about to say has no value, but..."

So, this post is "naught for nothing." You may get nothing from it, but you also didn't ask for it or pay anything for it... its nothing for nothing.

Most people who use it, though, likely just use it as a mix of verbal filler and a clumsy way to qualify their opinions to try to cut off any argument about what they are about to say.

I've always thought of the colloquial usage of this phrase as simultaneously self-effacing and gently but firmly assertive. "What I'm about to say may not be definitive or comprehensive, but quite obviously it merits consideration."

I've heard this phrase mostly on the east coast, and it often but not always has the same flavor as "just saying...." You're leading someone to an obvious conclusion but softening it slightly.

Let me add, I like the "naught for nothing" theory. In a bunch of the northeast "not" and "naught" would sound nearly identical, so now I'm questioning what I was actually hearing all those years.

'Not for nothing' has a different meaning in classical literature than it does in present day usage. In classical literature it means 'for a good reason'. For example, in the Jack London quote it means that the cub was exposed to the harsh realities of surviving in the wild in order to teach him those survival skills and increase his odds of survival when he's grown up and on his own.

In the present day it is almost always followed by 'but' as in 'not for nothing, but your well-trained dog is peeing on your carpet.' It may be used to gently suggest a course of action in a manner that allows the comment's target to save face (i.e. a polite way to say 'I don't think your plan will work. We should try this instead.'). More often it is used to point out an irony for the purpose of poking fun at or ridiculing the target, as in my example above.