Although often used as a greeting and often deployed at the beginning of a sentence, yo may also come at the end of a sentence and may be used for emphasis or to direct focus onto a particular individual or group or to gain the attention of another individual or group.
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, and later spread beyond Philadelphia.
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 late 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 supposedly 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. However, other sources state that Bush’s usage of the word "yo" is a “myth” and that the president actually said "Yeah, Blair".