For tis the sport to have the enginer Hoist with his owne petar.
Petard or petar was a container filled with explosives and placed against a gate to encourage it to open. The word itself comes from the Middle French word meaning to fart. So, what has this to do with hoisting anyone to his just deserts?
Unwelcome guests would place a petard on a long lever arm. The lever arm's mount resembled that of a trebuchet. The machine would sling the petard over against the gate and hold it in position while its operators remaining behind cover.
Upon release, the arm would hoist a careless, entangled army engineer toward the gate along with the petard. If this occurred at the wrong moment, it would result in the undoing of the engineer's structural integrity.
- Hoist with your own petard. The Phrase Finder. http://www.phrases.org.uk/meanings/hoist by your own petard.html.
- Petard. Wikipedia. http://en.wikipedia.org/wiki/Petard.