Never heard of the term........my guess it is a layman's term to mean what ever you want it to mean.
My guess is that it is as "solid as a Barn Door".....a massive heart attack with maximum heart damage.
It is a lazy, valueless, meaningless term in my opinion.
I also find it difficult to believe that it has any thing to do with horses and stable doors.
'Barn-door' is usually used as a reference to the great size of something, so perhaps it means a huge, usually fatal heart-attack rather than one from which it is possible to recover.
Whilst I am Not a medically qualified person, it is my understanding that 'Barn Door' is used in the Medical Profession to mean 'Obvious'.
In other words...It stands out as clear as a Barn Door.
medically I have never heard it but colloquially I have heard the description “xxx was like being hit by a barn door” so yes, sudden, massive and nasty
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