An acronym made famous by the Tiger-Blood drinking warlock himself, Charlie Sheen. He uses it in most of his tweets, in most cases next to different hashtags (e.g.: #winning #sheenius and so on), and the supposed meaning is either "You'be Been Warned" or, more likely, "You'll Be Winning".
Actual Charlie Sheen tweet:
@charliesheen Charlie Sheen
I am not only the book of Sheen-yuss but also the Book of Face.... yuss. go there now. *link* ybw