Petardʹ

Hoist on his own petard. Caught in his own trap, involved in the danger he meant for others. The petard was a conical instrument of war employed at one time for blowing open gates with gunpowder. The engineers used to carry the petard to the place they intended to blow up, and fire it at the small end by a fusee. Shakespeare spells the word petar: “ʹTis the sport to have the engineer hoist with his own petar.” (Hamlet, ii. 4.)