To "have a glass jaw" is a common saying in boxing. Basically it means you can't take a punch. And it's potentially a career-ending problem for a fighter. But it's been a mystery in combat sports for as long as fighting has existed: Why can some fighters take a punch and others can't?
The simple answer is: We don't know. What we do know, however, is that a knockout is usually caused by the head being whipped around by a blow to the chin or jaw. But while we can't really strengthen our chin per se, we can strengthen the neck muscles to allow us to absorb hard blows.
Yet we often see fighters focusing on the wrong neck muscles. Obviously, you want all of the muscles to be strong, but very often, fighters focus on the muscles in the back of the neck and the trapezius, which doesn't do much for a fighter, since approximately 70 percent of neck muscles sit in front of the neck, just below your chin.
If you think about it, wouldn't you want to train the muscles that are directly supporting the area that most often causes the knockout, aka your chin? Those muscles are called the deep anterior neck muscles. Here's how to strengthen them.
During all exercises, tuck your chin back, as to create a "double chin." By doing so, you activate the deep anterior neck muscles.
Lie down but make sure that your head is off the ground. You can do so by simply lifting your head off the floor, but for more range of motion, lie on the edge of something, such as a boxing ring or a bed, to allow for more movement of the head.
Make sure every movement is done in a controlled manner. So don't "shake" your head, but rather control your movement through the full range of motion.