Everyone knows that Japan has an ever-changing vocabulary of slang. It`s probably impossible to keep up-to-date on them all. I would like to introduce one that was popular maybe 2 years ago, which is KY, or 空気読めない. Basically, a KY person is someone who doesn`t `get it` or has no clue can`t read the signs from the current situation. For example, if a guy likes a girl and the girl is totally not into him, but he still thinks he has a chance and is persistent, he can be called a 空気読めないヤツ. When you type it, you just say he is KY. Or if that person already knows what that is, you can write SKY (SUPER 空気読めない) <– kind of sad, isn`t it, haha.
If you want to know all the new slang, here is a Japanese urbandictionary-like site: http://zokugo-dict.com/