According to my niece, "Yeah, no" is popular now w/ zennials. Also the laughing crying emoji is OUT, to be replaced with the hipper skull emoji to show amusement. As in, this is so funny it killed me.
"Yeah, no" was definitely a thing when I was in high school, and that was 10 years ago at this point. Maybe it's one of those all modern youth culture flows from the US West Coast things.
For example if someone asks me if I want to go somewhere with them at a specific time, and I would like to go with them but I already plan to do something else at that time, then I would say “nja” and while I say that think out an alternative suggestion for when we might go.
Another example is if I talk with someone and they say something that I agree with but then say something I disagree with. Then I would say “nja”. In this case it would be similar to English where you might say “yes and no”. And then state what I agree with and what I don’t agree with and why.
I would say /care, I don't care, either way, both fine with me, or "its nuanced" or "its complicated". The latter two invite curiosity and discussion, whereas the former one and its, ehh.., relatives (for lack of a better word) are more firm in the carelessness aspect therefore being more passive from my part. You can also say no and nod yes, as Torvalds famously did in response to a question concerning NSA.
I like the "yeah nah" expression! (It's best when accompanied with a head nod and shake when it's said.)
It's saying "I hear what you are saying, but I don't agree" with typical Aussie friendly honesty, with the implication that the proposition was unreasonable (often laughable).
Example, "Willing to swap late model Honda Civic for Ferrari or Lamborghini, must be road worthy and registered, cash my way if needed." Yeah, nah.
