The term "y-o" (pronounced why-o) is often heard on the Sopranos, i.e.
"a couple of y-o's came by to drop off a t.v."  It seems to refer to
someone as a jerk or moron.  It's also a real New Jersey street slang
term.

What is the origin of this term?

Hi Danibeeee!
Thank you for your question.
The "Whyos" (“Why-oh’s”) were one of five of five major gangs in New
York City, specifically Manhattan, in the late 19th Century. Though
"Whyos" is not an Italian-based word, it was used frequently in the
show beginning in the third season.
"In the period between 1880 and 1920, five major gangs controlled the
underbelly of Manhattan. These gangs were the Whyos, the Hudson
Dusters, the Five Pointers, the Eastman Gang, and the Gophers. Other
gangs existed in Lower Manhattan – the Hartley Mob of Houston and
Broadway, the Molasses Gang and the Mackerelville Crowd to name a few
- but they were rather minor in comparison to the numbers and
operations of the substantial five.(..)
The name	came from the gang’s cry, which, to an uneducated bystander,
sounded like a bird or owl calling, “Why-oh!”
http://www.geocities.com/rivercatsgang/facts.htm
"The Whyos consisted of several hundred members and were believed to
be the most vicious and terrifying of the time. They were so brazen in
their criminal activity that they printed up a list of services,
"Punching - $2, Both Eyes Blackened - $4, Nose & Jaw Broken - $10,
Jacked Out (knocked out with a blackjack) - $15, Ear Chawed Off - $15,
Leg Or Arm Broken - $19, Shot in The Leg - $25, Stab - $25, Doing the
Job (murder) $100 and up." Several years later [after the civil War]
the gang required all prospective members to have committed a murder
before receiving admission into the group."
http://www.knowgangs.com/gang_resources/gangs101/info_003.htm
In "The Soprano's: A viewer's Glossary", you'll find a guide to
Italian pronunciation, a glossary of terms and other interesting
tidbits about the show:
http://home.att.net/~ggjag5/glossary.pdf
   
I hope you find this information useful!
   
I very much enjoyed researching your question.
Thank you for using "Google Answers"!
regards,
~jackburton
  
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Search terms used on Google:
"couple of why-os"
"the sopranos" "y-oh" 
"the sopranos" "why-oh" 
whyos
whyos gang

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Clarification of Answer by jackburton-ga on 20 Jul 2003 03:23 PDT

If the link to the Sopranos glossary does not work, then you can view
the (HTML) file here:
http://216.239.39.104/search?q=cache:1pGnv_BXUngJ:home.att.net/~ggjag5/glossary.pdf+%22A+viewer%27s+Glossary%22&hl=en&ie=UTF-8
You will find the term "Whyos" listed at the bottom of page 7 of the
glossary. Even though "Whyos" is listed as a slang term, like
Pinkfreud, i also find it peculiar why the characters in the show
would make references to this gang. However, seeing as the "Whyos"
were a thuggish Irish immigrant group with their own slang and
customs, (and the first to advertise a price list for their violent
services!), perhaps this expression is used to refer to anyone who is
brash, brazen or socially undesirable.
As Pinkfreud pointed out, "Y-O" could also be a reference to "Youthful
Offender".
http://www.ci.nyc.ny.us/html/djj/html/jovsjd.html#youthful

I greatly doubt that Tony Soprano would make obscure references to
turn-of-the-century Irish gangs. Tony ain't no Martin Scorsese, if you
get my drift, ;-)

It is my belief that "Y-O", as used in "The Sopranos", is slang for
"youthful offender". "Y-O" is more or less a synonym of "punk kid", or
what used to be called a "juvenile delinquent" (which turned into the
abbreviation "J-D" in common parlance).

A Google search yields several hundred references to this use of the
term:

://www.google.com/search?hl=en&ie=UTF-8&oe=UTF-8&q=%22yo%22+%22youthful+offender