I've always wanted to know what a "petard" is, and why is it bad to get hoisted by it? I'm just a simple USAF guy, we don't "hoist" by "petards", we "nail 'em" with the "UCMJ". Gotta love the Navy pukes, with the ranks and language from the 16th century! ;)
:) Great.
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).