Basically, a petard was a mideaval bomb that was attached directly to castles to blast openings. They were often screwed in from a hoist apparatus to get at weaker areas of the walls and gates.
The term 'hoist by one's own petard' seems to refer to a soldier getting blasted up when his own bomb went off early (e.g., Max Clelland).
Thanks! I am always looking for some more knowledge of the language I use daily, and poorly. I wasn't trying to be a smartass, well, not much any way. I always like giving my brother/sisters in arms of the other services some good natured ribbing. Thanks again for the definition and where the phrase came from.