Some people just can't take a hit to the jaw. It's something about certain people that the acceleration of the skull after being hit in the jaw, the brain rebounds in the skull and hits hard against the inside of the bone. Some people's heads are more robust and don't have this susceptibility.

The literal meaning is that when they get hit in the jaw, they crumble to the ground as if they were made entirely of glass.

The term 'glass jaw' is a slang term for somebody that can be knocked out easily.

Pesonally, I think it's an unfair term. Anybody gets hit on the button will go down - period.

Nobody ever makes comments about glass ribs - A big fight decider.

Nobody ever passes comments on glass lungs - The No.1 fight decider... Guys that run out of gas..

Glass jaws is a phrase usually used by couch potato's.. Guys that have never set foot in a ring..

Go between the ropes... And, very few will mention glass jaws.

Of course, a lot of fighters get broken jaws so a glass jaw may literally mean jaws that break easily BUT in boxing parlance, it means the fighter gets easily kayoed from a hit to the jaw.

Al a glass jaw is and or means is that you just can't take a shot to the head with out either being knocked down or out. It means that yo just cannot absorb much punishment to the head

A "glass jaw" as properly defined in boxing is someone with a suspect chin who can not take a good punch if he's punched hard enough square in the jaw.

Glass Jaw means your jaw is weak and as soon you get hit on you're chin you are going to fold fast than a lawn chair.

You just pretty much answered your question...

Does it refer to your jaw physically breaking easily? Or does it refer to how easy it is to knock you out? But if so, what does that have to do with your jaw? Does have a susceptible jaw somehow make it easier for you to be knocked out for some reason and why?