hoist with one's own petard

Caught in one’s own trap, defeated by one’s own weapons. The term alludes to an ancient weapon, a thick iron canister filled with gunpowder, which was fastened to a gate or other barrier in order to breach it. It was a dangerous weapon, because the engineer who set it off could easily be blown up (“hoist”) when it detonated. Shakespeare was among the first to transfer the term, in Hamlet (3.4): “Let it work; for ’tis the sport to have the enginer hoist with his own petar.”