Is simply someone who has a chin that has grown into their neck. Or Neck grown into their chin. A double chin that has transformed itself to grow down into ones neck will pass as well.
These people who have a chneck have no profile of a defined chin. It is really just face and chneck.
A great example of a Chneck is Matthew Perry "chandler" of friends,he has a decent chneck. But there are so many people that you probably know yourself with a fabulous chneck of their own!