To be hoist by one’s own petard

On top of which it annoys those of us who really do know everything.

Dear Word Detective: After an exhausting weekend with a friend who knows EVERYTHING, I would be very grateful if you could give me the meaning of the phrase “Hoist one’s own petard.” Does it come from one of Shakespeare’s plays, and what is the meaning of “petard” — is it a sword, or is it a weapon from the Middle Ages? My honor is in your hands. — Mangisafi.

Exhausting is right. Have you ever noticed that those know-it-alls usually begin their sentences with the word “actually” to let you know that whatever you just said is complete nonsense, probably something you read on a cereal box or overheard in line at the 7-11? “Actually,” they sneer, “comet Hale-Bopp is made of ice and dust, not lint, and, being millions of miles away, cannot possibly be clogging the fuel pump on your car.” Sure, right. Like they know all about what certain comets can do.

In any case, I hope I’m coming down on your side of the argument when I tell you that a “petard” was a medieval weapon, specifically a small bomb used to blow open the gates of a castle under siege. The word “petard” (you can reveal this oh-so-casually to your friend) comes from the French word for “to break wind.” Petards, handy tools for those in the looting and pillaging business, did have a down side, however. They sometimes malfunctioned, “hoisting” (blowing skyward) the “engineers” delegated to plant the devices.

The phrase you’re thinking of, by the way, is “to be hoist by one’s own petard,” and does indeed come from Shakespeare, Act III of “Hamlet” to be precise. Hamlet, having sidestepped an assassination plot by having the unwitting bearers of the assassination order themselves “whacked,” muses on the justice of the moment: “‘Tis the sport to have the engineer hoist with his own petard.” This metaphorical use of the phrase to mean “someone being ruined or destroyed by the very plans or weapons they intended to use on someone else” has been popular since Shakespeare’s time. Oddly enough, the only modern example of the “hoist by one’s own petard” phenomenon that I can think of at the moment would be those cartoons about the roadrunner and the homicidal coyote. Not exactly Hamlet, I’ll admit, but there you have it.

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