safetype is quite right.
when I think of coffin corner, I think of the U2 spyplane and a very narrow gap between stall and overspeed.
most of us will not fly the U2.
Most of us will fly very docile transports and we shall try to keep them in the meaty part of the envelope.
sonic barrier
thermal thicket
coffin corner
are terms most of us will never really worry about. unless we don't know much.