The expression ‘hoist on his own petard’ comes from the frequent and generally grisly end of His Majesty’s Petardier—from the French, “person who has to light a very large fire-cracker at point-blank range while standing under boiling oil in a haze of arrows and musket balls”. The purpose of the petard was—at least in theory—to blow a bloody great hole in a castle door (6-inch-thick, two-hundred-year-old-oak beams held together with cast iron straps etc. you know, that sort of thing) and it was ignited like a cannon (ie by standing next to it and lighting a fuse).