Definitions by IFLEMV

Closed Mouth Smile (CMS) 

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)."

Also a noun:

"That is one creepy looking CMS."

Closed Mouth Smile (CMS) by IFLEMV September 12, 2013