The term ‘blown away’ has been around for close to 500 years.
Blowing from a gun is a method of execution in which the victim is typically tied to the mouth of a cannon and the cannon is fired. George Carter Stent describes the process as follows:
The prisoner is generally tied to a gun with the upper part of the small of his back resting against the muzzle. When the gun is fired, his head is seen to go straight up into the air some 40 or fifty feet; the arms fly off right and left, high up in the air, and fall at, perhaps, a hundred yards distance; the legs drop to the ground beneath the muzzle of the gun; and the body is literally blown away altogether, not a vestige being seen.
Read further: http://en.wikipedia.org/wiki/Blowing_from_a_gun