A petard is a small explosive device used in Elizabethan times to blast open doors or gates, much like a small shaped charge of plastic explosive might be used today. Thus, to be hoisted by your own petard is to be blown up by your own bomb. The word "petard" itself comes from the French and means literally "to fart". (Which would lend an entirely different meaning to being "hoisted by your own petard".)
From act III of Shakespeare's Hamlet:
For 'tis the sport to have the engineer
Hoist with his own petard: and 't shall go hard
But I will delve one yard below their mines,
And blow them at the moon: O, 'tis most sweet,
When in one line two crafts directly meet.
For more about the French and their fondness of farting, refer to http://www.manbottle.com/trivia/Joseph_Pujol.htm
WHO GOT IT RIGHT: Denny Maxwell, Dr Feelgood, George Oldroyd, Pete Daggett, JP Weigt, Iron Saul, Robert Walker, Robbie Moore, Kristy Morgan, Moana Kutsche, Bob Milligan, Trish Boggs, Fred Holcomb, Marika Thiessen, Paul Squires, and Joseph Tufano.