As I remember it JL is correct. I believe it was 'born' when a straight-wing twin jet US bomber came into service (anorak alert!-cannot remember the number, but it was before the B47 I think - could have been B45, and could have been a 'Martin')
and it could climb to this point. When crews tried to descend having realised their mistake the speed naturally increased as soon as they lowered the nose putting them at MCrit (shock stall, as it was known), which with a critical straight wing caused hideous wing drop and loss of control so I guess they just had to stay up there all day!