E. Cobham Brewer 18101897. Dictionary of Phrase and Fable. 1898.
Hoist with his own petard. Beaten with his own weapons, caught in his own trap. The petard was a thick iron engine, filled with gunpowder, and fastened to gates, barricades, and so on, to blow them up. The danger was lest the engineer who fired the petard should be blown up in the explosion.