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".
Etymology and history
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 was (and is still to this day) used often to get the attention of another person. The Middle English term originated as a variant of ya/ye ("yea, yes"), which is derived from Old English ġēa ("yea, yes"), which is derived from Proto-Germanic *ja ("thus, so, yes"). Thus, "yo" is etymological twins with "ya", "yeah" and "yea".
Whilst the term has always had some use in the English speaking world since the 15th and 16th centuries, its current popularity seems to have largely grown out of its use in Pennsylvania, particularly by 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.
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.
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.