In the 1940’s as aircraft were being flown closer and closer to the speed of sound, they began experiencing strange characteristics. Aircraft buffeting similar to a stall, aileron vibrations or “buzz”, controls shaking, losing effectiveness, or even locking up. People were really scared of this “Deamon that lived in the air.”
Coffin Corner – “The flight regime where any increase in airspeed will induce high-speed Mach buffet and any decrease in airspeed will induce low-speed Mach buffet.” – FAA Airplane Flying Handbook
From a pilots point of view coffin corner (also called Q-Corner or aerodynamic ceiling), is when the high-speed limit and low-speed limit on your airspeed come together as your altitude increases.
In the below image you can see the high speed and low-speed tapes converging.
“There was a demon that lived in the air. They said whoever challenged him would die. Their controls would freeze up, their planes would buffet wildly, and they would disintegrate. The demon lived at Mach 1 on the meter, 750 miles an hour, where the air could no longer move out of the way. He lived behind a barrier through which they said no man could ever pass. They called it: The sound barrier.”