Mixomax..
The term "coffin corner" refers to the point in an aeroplanes' performance envelope where the stall speed has increased to the same value as the Mach buffet, due to an increased altitude. In practical terms, the aircraft cannot climb any higher, regardless of power, due to the aerodynamic effects, thus it is sometimes referred to as the aerodynamic ceiling as opposed to the more normal (power limited) service or absolute ceiling.
Stall speed increases at altitude (more noticeable above 20,000')due to the reduction in Reynolds Number (crudely put - the viscosity of the air).
Mach buffet occurs as the airfoil accelerates the flow over the wing to the speed of sound, even though the rest of the aeroplane is at an indicated airspeed of less than Mach 1. The ailerons will possibly go to full deflection, rapidly each way (snatching), and Mach tuck (where the nose starts coming down regardless of the elevator position or airspeed) may occur, worsening the situation. Of course stalling an aeroplane isn't the best way to guarantee control over the next couple of seconds, either. In the coffin corner, on the recovery the aircraft may accelerate into Mach tuck.
At an altitude close the the limit, a turn will increase the effects, the flow over the faster (outside of turn) wing going more supersonic with more turbulence from the shock wave, and the flow over the slower (inner) wing slowing down. The machine is literally going both too fast and too slow at the same time - and it is only going to come down.
Either way, the aeroplane is not in controlled flight anymore - hence the term - coffin corner.
Common turbulence at high altitude can cause the same problems. Any commercial airliner pilot will compute (or have computed for him) the minimum and max airspeeds for the weight and altitude the airliner will be flying at, with an appropriate buffer for safety & comfort.
It is really only a problem for near gross weight, high flying jets, (or record setting gliders) the U-2 pilots were probably the first to name & popularise this situation, most previous jets being power limited in altitude.
Inappropriate commas, brackets and spelling due to lack of ability
, sorry