What's a petard, as in "hoist by his own ..."?
"Hoist by my own petard" — everybody says it, and so do I. But neither I, nor anyone else I've ever heard employ this particular cliche, has the slightest idea what a "petard" is. The one plausible explanation I've come across holds that a petard was a sort of 19th-century animal trap, a rope and a bent branch arrangement that caught the desired beast by one leg and pulled it up into the air. Can you confirm or deny?
Oh, I was supposed to pick one? Deny, of course. The line comes from Shakespeare, specifically Hamlet, act III, scene 4, lines 206 and 207: "For 'tis sport to have the engineer/ Hoist with his own petar …"
The Melancholy Dane is chuckling over the fate he has in store for his childhood comrades, Rosencrantz and Guildenstern, who are plotting to have him killed. Deferring his existential crisis for a moment, Hamlet turns the plot on the plotters, substituting their names for his in the death warrant they carry from King Claudius.
He continues: "But I will delve one yard below their mines/ And blow them at the moon." The key word is "mines," as in "land mines," for that's what a petard is (or "petar," as Shakespeare puts it — people couldn't spell any better then than now). A small explosive device designed to blow open barricaded doors and gates, the petard was a favorite weapon in Elizabethan times.
Hamlet was saying, figuratively, that he would bury his bomb beneath Rosencrantz and Guildenstern's and "hoist" them, i.e., "blow them at the moon." Dirty Harry couldn't have put it any better.
The word "petard," we note with a grin, comes from the Middle French peter, which derives in turn from the Latin peditum — the sense of which is "to break wind." Which must mean either that the French had a serious gas problem in those days, or that the petard was of something less than nuclear impact.