Source? Just watch this old Western movie from the 1940s sometime and tell me how many times you hear those cowboys use the word yo.;)   http://www.bluecorncomics.com/pics/yllwrbbn.jpg  I thought it was pretty funny when I seen John Wayne ride up to another guy on his horse and stick out his hand and say yooo :rofl I also found it amusing when they were reading off roll-call and all of the cavalry upon hearing their names read would respond with a hearty, yo! :D The word yo was used by cowboys and cavalrymen in the Old West, believe it or not.    Another seemingly unrelated version of the word yo was used by the Italian-Americans in large, east coast cities like Philadelphia and NYC as an interjection or as a synonym for hey and that is where the Blacks got it from. Blacks then popularized the word through it's constant use in rap songs by Black rappers from the east coast and the association between Black rappers and the word yo was really cemented by it's use in the all-rap MTV program YO! MTV Raps. :)