Hoist with his own petard

I have heard this phrase used often my whole life and today I decided to actually see what it really means. Since I used to be a rock climber who used ropes I had always thought it had something to do with ropes and climbing but I was wrong. It actually comes from Hamlet written by Shakespeare. Here are some quotes under the heading "petard" from wikipedia:

"Hoist with his own petard"

If a petard were to detonate prematurely due to a faulty or short slow match, the engineer would be lifted or "hoist" by the explosion. William Shakespeare used the now proverbial phrase "hoist with his own petard" in Hamlet.
In the following passage, the "letters" refer to instructions (written by his uncle Claudius, the King) to be carried sealed to the King of England, by Hamlet, Rosencrantz and Guildenstern, the latter being two schoolfellows of Hamlet. The letters, as Hamlet suspects, contain a death warrant against Hamlet, who will later open and modify them to instead request the execution of Rosencrantz and Guildenstern. Enginer refers to a military engineer, the spelling reflecting Elizabethan stress.

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 petar; and 't shall go hard

But I will delve one yard below their mines

And blow them at the moon: O, 'tis most sweet,

When in one line two crafts directly meet.

 

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A petard was a small bomb used to blow up gates and walls when breaching fortifications. The term has a French origin and dates back to the sixteenth century.^[1] In a typical implementation, it was commonly either a conical or rectangular metal object containing 5 or 6 pounds of gunpowder, activated with a slow match used as a fuse.

The word remains in modern usage in the phrase hoist with one's own petard, which means "to be harmed by one's own plan to harm someone else" or "to fall into one's own trap", literally implying that one could be lifted up (hoist, or blown upward) by one's own bomb.

 

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So when it says:

For 'tis the sport to have the enginer

Hoist with his own petar

 

What this actually means is: "It is sport when the military engineer is blown up by the bomb he has made." or translated it means when someone is planning harm that they are harmed by their own devices whatever they may be." So in common usage when someone says, "He is hoisted by his own petard it means that the way he planned to harm another went against the planner instead.

Another of life's many mysteries solved. If you are interested in learning more about this try "Petard" at wikipedia or at other sites on the web.