Last night I said to Garry “Aha! He is hoisted upon his own petard!”
By which meant he had just become the victim of what he (in this case a movie character) had planned for someone else. Then, I paused, thinking.
“What,” I asked Garry, “Is a petard?”
“I have no idea,” said my husband. Which is when I realized I’ve been using this expression my whole life and don’t know what it means.
Petard sounds French, but what is it? I grabbed my laptop and typed “hoist on his … ” into Google. Before I got to petard … up it came. Don’t you just love when that happens?
Voila! Courtesy of Wikipedia, here is the rest of the story.
A petard was a bomb used to blow up gates and walls when breaching fortifications. Castles. Walled cities. That sort of thing. The word was originally (duh) French and dates to the sixteenth century.
Typically, a petard was metal (bronze or iron), shaped like a cone or box. Filled with two or three kilos (5 or 6 pounds) of gunpowder and using a slow match for a fuse, the petard was a primitive, powerful and unstable explosive device.
After being filled with gunpowder, it would be attached to a wooden base and fastened to a wall, on or under a gate. The fuse was lit. If all went as planned, the explosion would blow a hole big enough to let assault troops through.
Thus the phrase “hoist on his/her own petard” came to mean “harmed by one’s own plan to harm someone else.” It suggests you could be lifted — hoisted — by your own bomb.