What is being "scared to death"?

Being "scared to death" can happen. The body's protective mechanism, the fight-or-flight response, pumps adrenaline into the blood, causing the nervous system to increase blood flow to muscles, dilate the pupils and, in some cases, evacuate the bowels. But adrenaline is toxic in large amounts and can cause death if it floods the heart unchecked. So being "scared shitless " isn't just hyperbole, either.