hoisted by your own petard

To be hoisted by your own petard means to be injured by the very device or weapon you meant to use to injure someone else. A petard was a small bomb used to blow up gates and walls when breaching fortifications and occasionally the person using the bomb would be injured by the backfire. The term being hoisted by your own petard relates to this in that when you try to injure someone else with a weapon, you in turn injure yourself.