A greeting, can also mean a Woman friend or associate that isnt intimate which in plural is "Yos"

Another way to say "hello", "hey" or "hi".

In the English language, yo has become a common interjection that originated decades ago in the Philadelphia region. It is often interchangeable with the word "hey," as in "Yo, what's up? or, "Yo! Wait for me!" While the word can also stand alone as a greeting, like the word "hey," it also has a wide range of meanings that depend on the tone, context, and situation in which it is used. (see examples)

In more recent times the word has spread into hip-hop or gangsta culture. A parody of such usage forms the basis for a comic exchange between the animated characters played by Robert DeNiro and Martin Scorsese (Shark Tale, 2004), which can be viewed here (http://yoism.org/index.php?id=YoST/).

Members of this later cultural scene may claim its origin as theirs. The word's origin, however, clearly predates these recent uses, as is evidenced by the words of Rocky Balboa (played by Sylvester Stallone)

"Yo, Adrienne, it's me, Rocky..." Rocky Balboa (Rocky, 1976)

"Yo, Adrienne. We did it!" Rocky Balboa (Rocky II, 1979)

1) Substitution for "Greetings!"
2) Dislexic Rabbi speak
3) Filler-speech when a need to look "gangsta" is at hand
4) Half of a "Yo-yo"

1. A phrase used to attract attention
2. The ghetto way of saying "Your"
3. A way to greet someone, like "Hey" or "Hi"
4. Is the spanish pronoun I

Word has it that "yo" started in South Philadelphia, the Italian area.

However this could be because of the movie Rocky, in which he lives in South Philly and says "yo" a lot.

The real history might be out there, but so far, this is the best guess out there.