Lockjaw is an old everyday name for tetanus, a serious illness you can get from certain bacteria, often through a dirty cut. The muscles tighten and will not relax; the jaw is the part people noticed first, so the mouth can clamp shut and eating or speaking becomes hard. That visible locking of the jaw is how the popular name was born. Doctors usually say tetanus; lockjaw sounds a little older and more informal, but English speakers still understand it. It is not a joke about a stiff face after a long yawn, and it is not the same as a jaw that clicks. A tetanus shot is the ordinary way people try to stop this before it starts.

破傷風（俗稱牙關緊閉）