I'm sure I could find the answer to this in a nanosecond by searching for it, but my guess it would be something to do with Rigor Mortis, or the peculiar stiffening of a corpse. If you were "scared stiff" you would be literally scared to death.
Phil
The cast of Scared Stiff - 1926 includes: Clyde Cook Stuart Holmes
The cast of Scared Stiff - 1997 includes: Casey McFeron as Stanley Becky Walters as Radio Announcer
Scared Stiff Alternative Medicine - 2010 was released on: USA: 30 September 2010 (internet)
The cast of Scared Stiff - 1919 includes: Eddie Lyons Mildred Moore Lee Moran Harry Nolan
Scared stiff
"Scared Stiff" (1953).
"Scared stiff" is an idiom that refers to being completely frozen with fear. It's origin is not known, but it has been in use since at least 1900 a. d. .
CSI NY - 2004 Scared Stiff 7-8 is rated/received certificates of: Netherlands:12
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Scared Stiff (1953)
Scared Stiff - 2013 Among Friends the Lobit Sisters - 2.0 was released on: USA: 20 August 2013
The correct spelling is "petrifying" (extremely frightening, as in "scared stiff").