petard

To be injured, ruined, or defeated by one's own action, device, or plot that was intended to harm another; to have fallen victim to one's own trap or schemes. (Note: "hoist" in this instance is the simple past-tense of the archaic form of the verb, "hoise.") I tried to get my boss fired by planting drugs on him, but I was hoist by my own petard when the police caught me with them beforehand.

Fig. to be harmed or disadvantaged by an action of one's own which was meant to harm someone else. (From a line in Shakespeare's Hamlet.) She intended to murder her brother but was hoist with her own petard when she ate the poisoned food intended for him. The vandals were hoist with their own petard when they tried to make an emergency call from the pay phone they had broken.

to be harmed by something that was intended by you to harm someone else The most enjoyable moment in any action film occurs when the villain is hoist with his own petard.

if you are hoist by your own petard, something that you did in order to bring you advantages or to harm someone else is now causing serious problems for you The government, who have made such a point of criticizing the opposition's morals now find themselves hoist by their own petard as yet another minister is revealed as having an illicit affair.

To be undone by one's own schemes.

Hurt by your own misdeed. A petard was a medieval bomb made of a container of gunpowder with a fuse, and to blow open gates during sieges against towns and fortresses. Unreliable, petards often exploded prematurely and sent the person who lit the fuse aloft (the “hoist” image) in one or more pieces. The phrase, which is often misquoted as “hoist on one's own petard,” comes from Hamlet: 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 . . .