Learn English – What’s the origin of “Copycat”

Hit Points as a way of referencing the general remaining health of a person is taken from the game Dungeons and Dragons, originally published back in the '70s. It was the first really popular stat-based role-playing game, and became kind of a touchstone for geekyness.

The idea was that you had a given number of hit points, based on your character class, constitution, and level. When attacked, you lose a certain number based on the power of the attack and a die roll. When/if your "HP" drops to 0, you die.

There are other games that use a "hit point" system these days, and probably were before D&D, but that is the game that popularized the concept. If you hear someone talking about "hit points" in real life, they are making a nerdy D&D reference.

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.