Learn English – Origin and meaning of the American 1960s slang phrase, “bread is”

I think that stack goes back relatively far in its meaning of "a large quantity". From the book Slang and its analogues past and present written in 1903:

The entry provides the phrase "stacks of the ready" to mean "plenty of money". I think this phrase, in the prevailing years, was shortened to the slang stack, which also took on the meaning of $1000.

Although I am loathe to use this as a source, Urban Dictionary lists that:

one stack = 1 G

That is, one stack is equivalent to one grand which is $1000. The use of stack in this fashion is very much slang, so it is not in more established dictionaries. However, given that Urban Dictionary is a crowd-sourced source which sometimes contains the most current uses of words, and that this meaning of stack is entered twice, I am sure that a stack is equal to $1000.

I think that any etymology of "Yo!" that goes back only a few hundred years is woefully incomplete and quite absurd.

"Yo!" is used in more-or-less formal situations in East Asia (China, Japan), India (Dravidian languages), Africa (West and Central Africa), the United States, and Europe. That usage range puts it well beyond the purview of Indo-European, and suggests that its origins could lie entirely outside any formal etymology - but if it does have an origin, it obviously ain't English (as your source up there says, suggesting it may have come from Africa, or the Mediterranean, or both).

Arguing that this simple sound is derived from "an exclamation" back in AD 1400 is saying nothing more than "Back then, in AD 1400, nobody knew where it came from, either." Compare, for instance, the exclamation "Zounds!", which has a certain date of origin, and a certain meaning from which it is derived: "Yo!" has none of that.

Basically, "Yo!" is a simple sound that gets used a lot, around the world; so long as it's not a formal word in one's local language, it will tend to get used for more-or-less formalized exclamatory purposes. This makes sense because it's A) easy to say, B) the sounds occur in pretty much any language on Earth, and C) the sounds carry a quite a way's distance, and are easily distinguished from other sounds and words.

In the US, it was re-purposed as a greeting and response by Af-American culture some time in the late 60's, or so, and that's the answer you really want, here. It may have been absorbed into Af-American culture through Basic Training in the US military, during Vietnam (or WWII, as suggested by the other poster, above), or it may be a holdover from something more ancient, perhaps an African dialect; it's to answer questions like this that the idea of "ebonics" was once promoted. I have no idea if that discipline -- if it can be called that -- is still around or not, but that might be a good place to start if it is. In any event, it appears that currently linguists just can't really give your question any definitive answer.