Shortened form of "y u no," itself a corruption of "Why don't you." Commonly used by l33t hax0rs and twitter users.
Pronounced "Why you no."
Internet user: Webcomics yuno bottomless?
Twitterer: @GBush yuno get impeached #medicare #yolo #iraq #winning
Hax0r: GENTOO YUNO HAVE FFMPEG!?$!?@#!1#?!
Another word for slut.
Man she was such a huge yuno.