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.
Originates from "you don't even know" & it's something you say in a smug sort of way highlighting the ignorance of the people present over something lit you just did or are still doing. Of cos, some kind of admiration or positive amazement has to obvious in the people's faces.
e.g people arguing on how some kind of dance is performed & you perform it to perfection with everyone stopping to watch...it's only right you smugly say U.O.E.N.O immediatelyyou're done!