What's the origin of “yo”?

Solution 1:

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.

Solution 2:

There are actually a few different meanings reported in the OED for "yo" as an interjection, or a response:

• An exclamation of incitement, warning, etc. (also repeated). In nautical use = yoho int. Occas. as n. and in vbl. n. yo-yoing.
  • first noted use: 1420
• An exclamation used as a greeting, to express surprise, or to attract attention; hey! Also as a response to such a call. slang (orig. and chiefly U.S.).
  • first noted use: 1958
• slang (orig. in African-American usage). In weakened use, following or punctuating an utterance for emphasis or as a general conversational filler.
  • first noted use: 1987

So the OED says that yo can be traced back to the 1400s, though not the 14th century. As an interjection, it has been used as a warning or emphasis, and weakened within the last 30 years. It has fallen into different groups (nautical, US slang, African American slang) but its current use remains tied to its 1400s use. As many words change in meaning over time, it seems well-founded to say that yo is from circa 1420.

It is possible that yo is even from the 14th century, and was only recorded in 1420. As a solid date for which the interjection was in use, however, 1420 is a sourced answer.