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

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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

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reduction of "Years Old"
12YO girl stabs sex-hungry boyfriend to death
by Mikemike February 21, 2006

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form of saying hello, or what's up?
yo!
by Jessica June 07, 2003

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The coolest Italian slang ever!
Mario: Yo, whattup Luigi!
Luigi: Yo!
by Italia Marzioli November 26, 2016

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Slang for you.
Yo! You going to that big party at Jake's house tonight?
by Anastasia Minette. April 13, 2016

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american slang short for "you" and is a way to get ones attention.
YO'! Where's my cheerios?!
by icoloreggswithsatan May 09, 2011

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