Why is tetanus also known as lockjaw?

Muscle spasms. Tetanus is caused by a bacteria called clostridium tetani. One of the initial symptoms that patients experience is spasm of the muscles that control the opening and closing of the jaw. The muscle spasms prevent the opening of the mouth, commonly referred to as lockjaw.
Jaw smasms. The toxin produced by the germ circulates in the blood and effects the muscle groups by causing tremendous spasms that cannot let up. This can lock the jaw closed. "lock jaw".