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"Hoist with his own petard"
From Wikipedia, the free encyclopedia
"Hoist with his own petard" is a phrase from a speech in William Shakespeare's play Hamlet that has become proverbial. The phrase's meaning is literally that the bomb-maker (a "petard" is a small explosive device) is blown up ("hoisted" off the ground) by his own bomb, and indicates an ironic reversal, or poetic justice.

The phrase occurs in a central speech in the play in which Hamlet has discovered a plot on his life by Claudius and resolves to respond to it by letting the plotter be "Hoist with his own petard." Although the now-proverbial phrase is the best known part of the speech, it and the later sea voyage and pirate attack are central to critical arguments regarding the play.The phrase, and its containing speech, exist in only one of three early printed versions of the play — the second quarto edition — and scholars are divided on whether this is indicative of authorial intent, or a mere artefact of playhouse practicalities.