Yo!

Yo is an English slang interjection, commonly associated with American English. It was popularized by the Italian-American community in Philadelphia, Pennsylvania in the 1940s.

Although often used as a greeting, yo may come at the end of a sentence, often to direct focus onto a particular individual or group or to gain the attention of another individual or group. It may specify that a certain statement that had been uttered is more important, or may just be an "attention grabber".

The interjection yo was first used in Middle English. In addition to yo, it was also sometimes written io.

Though the term may have been in use in the 16th century, its current popularity stems from its use in Philadelphia's Italian American population in the twentieth century, which spread to other ethnic groups in the city, notably among the African Americans.

From the late twentieth century it frequently appeared in hip hop music and became associated with African American Vernacular English, as seen in the title Yo! MTV Raps, a popular American television hip-hop music program in the 1980s.

A frequent example of the expression is fictional Philadelphian Rocky Balboa, where the word is used throughout all of the Rocky films, and is part of the iconic line, "Yo, Adrian, I did it!", which was ranked 80th in the AFI's list of 100 best movie quotes.

The phrase "Yo, Blair. What are you doing?" was an informal greeting that United States President George W. Bush gave to British Prime Minister Tony Blair during the G8 summit in Saint Petersburg, Russia, on 17 July 2006.

...
Wikipedia