When/where/why did the neck-swivel/finger wag originate?
I suspect it’s a strange anatomical interaction in humans - sort of like when I pet my cat in just the right way it performs a licking motion. I believe it was first documented in ancient Egypt as an attribute of the falcon-headed god Horus. But that’s just a guess. 
You mean the one that is often accompanied by “Oh, no you di und!”
Don’t lick a gift Horus in the mouse.
Yes, the one practiced primarily by Africo-American women. Finding a “first occurrence” might be dicey, but I’d say it gained currency from programs like “Jerry Springer” in the 1990s. At least that’s where I remember Ellen Degeneres making fun of one such subject when she guest-hosted “Talk Soup” back in the day.
So, this is one of the great unsolved mysteries of our time.
Oh, no you di’int.