Why not? You could compete and win in wrestling or jiu-jitsu and never have your chin tested. Some very good boxers were known for having "glass jaws' but their skills in slipping punches and movement never exposed it until they were deep into their careers and fought very skilled boxers. Amir Khan comes to mind. Some MMa and kick boxers developed glass jaws so to speak after being badly knocked out or after long careers of taking punches. Chuck Liddell, Mirko Cro Cop are a couple that come to mind. Great , amazing fighters but at the end of their careers both were labeled as developing "glass jaws'. The brain can only take so many concussions. Usually when a fighter is being labeled with a glass jaw though you must remember he is being hit by professionals and most of them hit like freight trains. Against an average Joe they would probably be fine.
Never heard it called a glass chin though.
