Saw or read this term somewhere recently and would like to know what it means.  I know it has something to do with buffetting
Cheers
Splash

Coffin corner occurs at an aircrafts absolute ceiling, where the speeds at which mach number buffet and prestall buffet occur, are the same

For a constant mach number, the IAS decreases with altitude due to decreaing LSS. To prevent the IAS from deacreasing to its stall speed, the mach number must be increased, which results in an increase in IAS.
But if you fly a constant IAS (in climb), the mach number will be increased to a point where it exceeds Mcrit, to prevent this from happen the IAS must be reduced.

So simply put, there comes a point at the aircrafts absolute ceiling where the aircraft can't go any higher. Because it is bounded  on one side by the low speed buffet and on the other by the high speed buffet because the stall IAS and Mcrit values are equal.

That, is the coffin corner...the effect restricts the altitude attainable by the aircraft.

okay...imaging you're balancing on a thin board:

It is a balance act allright...If you fall to the left you will fry in the fire of hell.
However if you fall to the right you will instead freeze to death in the cold ocean.

That's the difference...if you fly slower, you will drop out of the sky, if you fly faster some part of the airflow (if not all parts) over the wings will be supersonic, hence creating  a dramatic increase in drag coefficient.

So it's that very fine line between highspeed buffett and low speed buffet?  I can understand why they call it Coffin Corner.  Ah, and the Deadman Zone in a chopper.
Cheers
Splash

Splash, regarding the "line" between Stall and supersonic:
In real life, the mach number buffet and prestall buffet are difficult to extinguish between, that's why a margin is imposed between an aircraft's operating ceiling and its maximum (absolute) ceiling.