Back in the 70's and 80's, the baby boomer Italian's from NYC used "yo" like an equivalent of whistling out loud to get someone's attention-- usually someone at a distance or if you wanted to immediately get attention up close. The word was also preceded with a "hey" in many cases, as in "hey, yo." This was the old-Italian guard usage. When African Americans got a hold of it, "yo" was not only used with the "Italian usage," but also became an abbreviation for the possessive, "your."
Otherwise, see Lili2344 for a history lesson on where the slang originated.
Brooklyn vernacular:

Hey yo, Joey, wutsamatta. I ask'ja ta trow me the ball tree times already.

African American colloquialism: (now used by all ethnicities despite class, education, and geography)

Yo! Rick baby, what's with yo ride yo?
by Mark Kessel January 28, 2008
An ancient word that is inputted into any type of freakin sentence and still works and is used to make one sound gangsta but only makes people shake their head at u with pity.
"Yo, I dropped that. Get it for me, yo."
"YO!!! Where u BEEN, yo?!" *passersby shake their head with judgemental disappointment*
by Emily_aka_Amber June 27, 2016
reduction of "Years Old"
12YO girl stabs sex-hungry boyfriend to death
by Mikemike February 21, 2006
form of saying hello, or what's up?
by Jessica June 07, 2003
Slang for you.
Yo! You going to that big party at Jake's house tonight?
by Anastasia Minette. April 13, 2016
american slang short for "you" and is a way to get ones attention.
YO'! Where's my cheerios?!
by icoloreggswithsatan May 09, 2011
Used a million times by the next rapper in a circle to let you know it's your cue to stop.
*rapper 1 raps*
*rapper 2 says "yo" several times*
*rapper 1 stops*
*rapper 2 starts*
by Krome Dome February 26, 2008
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