The act of smiling without showing one's teeth so the lips are stretched wide across the face while remaining closed. If done correctly, looks mischievous, but mostly creepy.
"Okay for this pic, don't smile, let's Closed Mouth Smile (CMS)."
Boys who enjoy yaoi (a genre in Japan that contains sexual and/or romantic relations between two men); literally translates to "rotten boy"; corresponding female : fujoshi