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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 was previously uttered is more important, or may just be an "attention grabber".
The interjection yo was first used in Middle English, specifically in the 15th to 16th century. In addition to yo, it was also sometimes written io. It is often used to get the attention of another person.
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 1990s.
The popularity of the interjection "Yo" is believed to have grown out of Philadelphia's Italian immigrant population. During the mid-1940s, Philadelphia's adjacent Italian and African-American neighborhoods experienced significant growth, resulting in the merging of the two cultures. Tension between the two groups was notably high, which was fueled by a high crime rate, but certain cultural oddities passed between the two groups. The most influential of these transferences is the word "yo,", a term that possibly stems from the Italian first person singular "io."
In the Neapolitan dialect "Guaglione" (pronounced guahl-YO-nay) signified a young man.  The chiefly unlettered immigrants in cities such as Philadelphia and New York shortened that to guahl-YO, which they pronounced whal-YO.  This was possibly further shortened to yo.
A famous example of a fictional Philadelphian bringing notice to the expression is 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 summit of the "G8" (Group of Eight industrialized nations) in Saint Petersburg, Russia, on 17 July 2006. The quote gained wide popularity across the media, and many began to use the phrase in a casual manner, interjecting it at certain points in sentences, as it was popularized.
The term is now widely used amongst all ethnicities in the Philadelphia metro.