a conical or cylindrical case of metal filled with powder and attached to a plank, to be exploded against and break down gates, barricades, drawbridges, etc
If someone who has planned to harm someone else is hoist with their own petard or hoist by their own petard, their plan in fact results in harm to themselves. The students were hoist by their own petards, however, as Granada decided to transmit the programme anyway. (2)