I knew petard was a bomb. I fear my mistaken recall of "upon" is that of a child of the Space age, imaging the man sitting upon the bomb, lofted upwards by its detonation as if upon a rocket. I knew "peter" (peh-tay) was the verb for fart in modern French but hadn't made the connection to an Avonian double entendre. And am grateful for another mental image that evokes rocketry. The humble fart, both repulsive and propulsive.