As a sort of meaningless starter, “Yeah, no,” has become to go-to starting phrase of many guests on news shows, typically responding to the host/questioner making some sort of statement and then turning to the guest and soliciting an opinion. In the past, I think the guest might have begun his or her response with “Well” but now it’s the nonsensical phrase “Yeah, no,” even when the guest is expressing neither disagreement or agreement, much less both at once. When did this phrase arise, and why, do you suppose?
I don’t necessarily see it as nonsensical, it depends on how the host phrases the question. If the host is making an assumption in his question, then yeah,no makes sense. It means, yes I hear what you are saying, but no you got wrong.
It’s a talking trend. They come and go. I think it arrived because “I know, right?” with an upward inflection on “right” was abandoned.
It has been in Australian usage for as long as I can remember. Variations of this are traditionally employed by Australian test captains when dancing around an explanation of team performances
Maybe but i frequently hear “yeah, no, that’s exactly right.”
I think more often it’s used in the context of what Si_Amigo says, but it could probably also be used if someone says somethng like “is this thing I believe true, or am I crazy?” “yeah, no, that’s right, you’re not crazy”.
Using the words “yes” and “no” in the same sentence sounds like a politician’s dream. LOL
I’ve never heard “yeah, no” and then agreement. If I were to hear that I would assume the responder misspoke. Typically “yeah, no” means “I heard you, but I disagree and here’s why”
I first noticed this “Yes, no” idiom in a British sitcom called “W1A,” which came out in 2014 and is a satire of the BBC itself. Every character in the series said it, and it was said over and over again. I figured it was a phrase that was a part of British culture and was being used as part of the satire. Since then, it seems to have taken hold everywhere and like many of you, hear it quite often.
I hear it all the time. What it’s doing, I think, is just giving the responder some extra time before he gathers his thoughts. Sort of like, “Well” or “Errrr” or “Ummmm.” That’s why I called it “meaningless.”
My thread from 2011:
Before that, I recall it as the catchphrase for a Matt Lucas character on Little Britain. The annoying teenage girl.
In my world “Yeah, no…” means no. If you want to indicate agreement you say, “No, yeah…”.
I’ve heard more “Yeah, I mean…” when interviewees start talking: “Yeah, I mean, we got the votes to make it legal for anyone over the age of two to carry guns.”
On the other hand, I’ve only come across “Yeah, no” to shut someone down, meaning, “Not on your life, buster!” as in, “Yeah…no, I won’t go on a date with you Leo.”
It’s not all about you Leo. But yeah, no. I most likley wouldn’t date you either buts thats only based upon your profile age of 8. 
I don’t think I’ve ever heard it as a synonym for “well” or followed by agreement. It’s been a common phrase for a donkey’s age as far as I know, and I only remember hearing it the start of a contradiction, so far as I know.
That said, Wiktionary does give two definitions, so I’m guessing that it has morphed a bit over time.
Is it like this use of “yeah, no”?:
It comes out of the Lil’wrekkers mouth frequently.
I’ve used it a few times when I wanna be obstinate or non-compliant. But that’s just me.
Typical way I’ve seen it used on TV: Dorky guy asks out a pretty girl, and she responds, “yeah… no”. Definitely a put-down and a refusal. She might as well say, “Sure… when hell freezes over.”
The way I understand “yeah, no”, and the way I use it is as a more emphatic “no”—a sort of pseudo-mean spirited “no”, with a passive-aggressive overtone (but, somewhat tongue-in-cheek). It’s like sucking someone in, making them think you are agreeing with them, only to hit them with an emphatic, heart-breaking “no!”
Example:
Bob: Sally, do you think I’m sexy?
Sally: yeah,… 
Bob: [hopeful for a microsecond] oh good, Sally thinks I’m se…[/hopeful for a micro-second]
Sally: no 
Bob: [crestfallen forever after] ouch, that hurt. But touche for Sally, she zinged me pretty good [/crestfallen forever after]
Also, it’s important for the “yeah” to be said with a pleasant smile and ascending inflection, and the “no” must follow with an exaggerated frown and descending inflection.
If that’s not the current meaning of the term, then I propose we change it to that, because you can never have too many zingers at your disposal.
The first hit on Urban Dictionary is from 2004, meaning “a smooth way to tell someone fuck no.”
The second is from 2008 and is suitably cranky. “A phrase that people now use to start sentences for some goddamn reason.”
I never heard anyone say it in real life, though. Only on the internet.