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1.
UOENO is a corruption of the phrase "You don't even know". Colloquial vernacular predominately used in the south eastern and central southern portions of the United States. It is an expression of someones' lack of knowledge about a certain situation, circumstance, person, or matter. Can also refer to the degree that someone lacks knowledge concerning a situation, circumstance, person, or matter.
My boss made me so mad to that I was ready to quit. UOENO!

also

I heard that Tami was hitting on you. (reply) UOENO
by Urban Etymologist for Hire April 20, 2013
 
2.
You don't even know
"This a thousand dollar pair of shoes and UOENO it
This a thousand dollar cup of lean and UOENO it"
by Aine Treblig August 20, 2013
 
3.
“U.O.E.N.O” is onomatopoeia for “you don’t even know.” It’s somewhere between an abbreviation and an acronym.

Hint: Say it out loud, quickly.

Made popular by the song "U.O.E.N.O." By Rocko Ft. Rick Ross, A$AP Rocky, Wiz Khalifa and 2 Chainz.
This a thousand dollar pair of shoes and U.O.E.N.O it

This a thousand dollar cup of lean and U.O.E.N.O it
by peptobizniz July 05, 2013
 
4.
stands for "You ain't even know it." If you say the letters aloud it will help you say "You ain't even know it" with an urban accent. This term is used when you are doing something so epic, people don't even realize it's right in front of their face.
These shoes cost half a million dollars, U.O.E.N.O.
by MS3MY July 08, 2013