Abbreviation for "You know what?"
A term used by people who have decided that it's too tedious and un-cool to spell out the abbreviated form of "you know what" with the actual first letters of each word.
A: no, i asked you first.
B: Asked me what?
B: WTF ARE YOU TALKING ABOUT
It means "you know who" and it is used when you dont specify the name of the person but the person who you are talking to or the peron who is reading your message would understand who you are talking about.
Mary told Joanne, "9'o clock.Ukw
is checking me out"