What does 'hoist by own petard' mean?

The petard was the first 'shaped' explosive charge, used by a Mining Engineer (colloquially 'sapper') to precisely direct a blast onto a single point, like a lock or door hinge, when mining toward a defended structure during a siege. It was shaped like a squat cone and because of this shape, concentrated a small charge so well that the sound of the blast rarely carried through the door to the defender. This enabled the sapper to use such a small charge that he could remain quite close to the spot. So close, in fact, that it was conceivable that a sapper might be 'hoist by his own petard' if he was a bit of a show-off. First printed reference was in Shakespeare's 'Hamlet' III.iv.207 "For tis the sport to haue the enginer Hoist with his owne petar"