This is an acronym for You Know You're Obsessed When...
This is usually shortened in threads so that a registered user of any given forum (for fans of either bands, celebrities, or the like) can talk about their list of obsessions without having to spell the whole acronym out.
Then, the registered users of said forum
will spend pages upon pages describing a tattoo, sweater, hair part, etc. that whoever/
whatever they're obsessed with has. And, of course, the rest of the site members
will join in to say that the part is actually on the other side, and that you're a fool for not knowing and then you
will immediately be shunned from that site for not being a
TRUE *cough*stalker*cough* fan.
User1: YKYOW you have a vile of Angelina Jolie's blood!
User2: haha You have one too? lolz! we so
cool.
User1: *air five!*
User2:yeah!
OR
YKYOW you stalk Tegan and Sara's tour
bus just so you can get them to take a picture with you and your chest tattoo of
Tegan/Sara's face.