A petard was a medieval term for a small bomb used to blow up gates and walls when breaching fortifications.

It remains in modern usage in the phrase to be hoist by one's own petard, which means to be harmed by one's own plan to harm someone else. Shakespeare used the now proverbial phrase in Hamlet.

In the following passage, the "letters" refer to instructions 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. Engineer refers to a military engineer.

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 engineer
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: O, 'tis most sweet,
When in one line two crafts directly meet.

After modifying the letters Hamlet escapes the ship and returns to Denmark.

The verb "hoist" is an irregular past tense of the obsolete verb "hoise," meaning "raise" or "lift." The same form is used in "burn" and "burnt."

The phrase is usually misquoted as "see the engineer hoist by his own petard" and is taken to mean "the hangman hanged with his own rope," or, as in Roadrunner cartoons, a rope, put out to catch something, entangles and hangs the one who set the trap, while the audience "sees" (watches) in amusement.

Hamlet's actual meaning is "cause the bomb maker to be blown into the air with his own bomb," metaphorically turning the tables on Claudius, whose messengers are killed instead of Hamlet.

A modern (1966) play "Rosencrantz and Guildenstern are Dead" was written by Tom Stoppard, containing (imaginatively) all that happens to Rosencrantz and Guildenstern off-stage while Hamlet is occurring on-stage, including this journey to England.