Why's "what's up" used as greeting among young people? It sounds like asking "what is up there"?
A: What's up?
B: The sky.
Learn English – Why “What’s up”
etymology
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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.
The term “the pot calling the kettle black” is usually used in the sense of accusing someone of hypocrisy. The origins of the phrase date back to at least the 1600s, when several writers published books or plays which included wordplays on this theme. Despite suggestions that the phrase is racist or nonsensical, the meaning is actually quite obvious when one considers the conditions of a medieval kitchen.
Typically, pots and kettles were made from heavy materials like cast iron to ensure that they would last and hold up to heat. Cast iron tends to turn black with use, as it collects oil, food residue, and smoke from the kitchen. Both pots and kettles would also have been heated over an open fire in a kitchen. As a result, they would have become streaked with black smoke despite the best cleaning efforts.
Since both are black, the pot calling the kettle black would clearly be an act of hypocrisy. The act could also be described by “it takes one to know one,” and it suggests a certain blindness to one's personal characteristics. There is another explanation for the term, involving the pot seeing its black reflection reflected in a polished copper kettle. In this sense, the pot does not realize that it is describing itself.
One of the earliest written instances of the phrase appears in Don Quixote, by Cervantes. The epic book was published in the early 1600s, and had a big influence on the English language. Numerous terms and idioms have their roots in Don Quixote, such as “quixotic” to describe an idealist. Shakespeare also played with the concept in one of his plays, as did many of his contemporaries. The phrase has been twisted and expanded over the centuries, appearing in forms like “pot, meet kettle.”
Some people believe that the phrase is racist, since it refers to the surface color of the objects involved. These individuals might want to keep in mind that in a modern kitchen, the idiom might be “the pot calling the kettle silver,” in a reference to the fact that many modern pots and kettles are often made from polished stainless steel. In this particular instance, skin color has nothing to do with the idiom, except in the sense that both of the objects involved are the same color.
Phrases.org.uk defines its meaning as:
The notion of a criticism a person is making of another could equally well apply to oneself
Best Answer
Up has many, many uses and referring to something that is above is just one of them.
What’s up is not an innovation by the young of today. As a statement or a question about what might be happening, it isn't particularly new. In a letter written in 1838, the novelist Elizabeth Gaskell wrote I did not mention a word to Lucy but she must have guessed something was ‘up’. Even in its reduced form of wassup, it’s over 100 years old.