The phrase “hoisted on his own petard” means to be harmed or ruined by one’s own plan or device that was intended to hurt someone else. It comes from a line in William Shakespeare’s play Hamlet, where Hamlet says that he will make his enemies “hoist with their own petard”. A petard was a small explosive device used to break down walls or gates in warfare. To be hoisted with one’s own petard means to be blown up by one’s own bomb. It is an example of poetic justice or irony.
