petard: This word really only survives in the phrase "hoist with his own petard." A petard was a small bomb used by medieval engineers to breach castle walls or bring down drawbridges. It comes from the French word peter, "to break wind, toot, fluff, fart." (So if a Frenchman laughs at you because your name is Peter, now you know why.) JRR Tolkien fans might remember that the Deeping-wall of Helms Deep was opened by a suicidal Uruk-hai toting a petard.
Long before the creation of Middle-Earth, the word petard (indeed, the well-known phrase) appeared at the end of Act III, Scene 4 of Hamlet, after he had inadvertently murdered Polonius and had reminded his mother that he was to leave for England with Rosencrantz and Guildenstern:
There's letters seal'd: and my two schoolfellows, —
Whom I will trust as I will adders fang'd, —
They bear the mandate; they must sweep my way
And marshal me to knavery. Let it work;
For 'tis the sport to have the enginer
Hoist with his own petard: and 't shall go hard
But I will delve one yard below their mines
And blow them at the moon . . .
Hoist with his own petard now means, of course, to be injured or killed by one's own schemes. Or maybe it refers to accidentally catching one's pants on fire when lighting one's farts.