Hoist me petard, me hearties

"Pray tell, what is the origin of hoist on [his] own petard?" asks Joe Griffin . "I've been led to conclude that it may have to do with the Greek legend of Herakles and Prokrustes (sp?) but I'm not sure."
    And Michael Annett of New York, wrote many moons ago in a message which had been lost in the bowels of my hard disk until I started digging out the answer for Joe:
    "An associate of mine suggested that I had been hoist by my own petard when I managed to 'trap myself in my own web' so to speak.
    "I questioned him on the meaning of this phrase and he explained that it had something to do with English sailors and raising flags. I consulted an American dictionary which defined petard as a flatulent report and it seems to me that the phrase means to be raised up using intestinal gas as a motive force. Can you shed any light onto the origin of this phrase?"

Like Michael's associate, I thought for years that a petard was naval - a yardarm, perhaps, or a sail. I had visions of unfortunate sailors being yanked into the sky as they tried to trim the sails, or of even more unfortunate sailors being hanged from the yardarm. The misapprehension was worsened by the common use of hoist on rather than hoist by.
    Hoist means "raised", but in this phrase it has nothing to do with pulleys or sails or ropes or masts: it means "blown up". And petard has a great deal to do with "fart". Or did originally: the Indo-European wordstem perd- eventually produced the French peter, to fart, as well as the English fizzle and petard.
    A petard was a military mine (which presumably produced a loud fart-like noise when it exploded) which was often used to smash castle walls during sieges. As with many things mediaeval and military, petards were unreliable: they could fizzle out, or they could detonate before the soldier had a chance to leave the area. Such a soldier was hoist by his own petard.
    I suppose that if a petard were built in the shape of a Santa Claus (not something that serious soldiers would do, I admit) it could be called a Farter Christmas.
    Sorry.
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