This is the latest in self-improvement techniques that has become the thing to do on the internet. The trick of mewing is to flatten your tongue against the roof of your mouth. This pushes your bottom jaw forward so that you lose the receding jaw and the double chin. It also improves the alignment of your upper and bottom teeth and encourages you to breath through your nose. There are before and after shots on the internet to show you it can be done, although people who are serious about it warn that it doesn’t happen overnight.
The British dentist, Dr Mike Mew, is credited with spreading the word on mewing although it was his father, Dr John Mew who created the field of othotropics in which the teeth are regarded in the context of the whole face. The Mews talk about facial growth guidance rather than mewing, but the popular name has taken over. There is even the verb to mew.