Mewing involves keeping your tongue pressed to the roof of your mouth to change your jawline's shape. When someone tries talking to a person that is mewing, the person will put their index finger on their mouth (the 'shh' gesture) and then put their index finger on where their jawline starts and run it down their jawline.
ex. "Dude! Did you see John Doe's jawline in the grouppicture?"
"Yeah! I heard he's been mewing!"
(verb, orthotropics) a form of oral posture training to improve jaw and facial structure. It was named after Mike and John Mew, controversial British orthodontists. It involves placing one's tongue at the roof of the mouth and applying pressure, with the aim of changing the structure of the jaws.