Hoist with his own petard
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, an't shall go hard
But I will delve one yard below their mines
And blow them at the moon.
"Hoist with his own petard" literally means "blown up with his
own mine." More generally, a "petard" is a hat-shaped device which
can be be charged with gunpowder. Here's how Hamlet arrives at the
The prince—having "caught the conscience" of the murderous King
Claudius [see THE PLAY'S THE THING]—is about to be packed
off on a little vacation to England. He apparently hasn't been
feeling too well in the head, so the king orders Hamlet's old
school chums Rosencrantz and Guildenstern to keep him company.
Though the two fools are relatively innocent (for spies), Hamlet
knows that Claudius has some "knavery" planned and that Rosencrantz
and Guildenstern are the king's tools.
Hamlet has already guessed that the letters his schoolfellows
are carrying to the English court bear a "mandate" for his
immediate execution. The prince intends to hoise this mine in the
king's face. Claudius is the "enginer," the deviser of the
explosive; and his plot is about a yard shallower than that of his
nephew, who will send Rosencrantz and Guildenstern to the fate
intended for himself.