Etymology Today

In case you've never looked up the origin of this word:

Main Entry: pe·tard
Pronunciation: p&-'tär(d)
Function: noun

1 : a case containing an explosive to break down a door or gate or breach a wall
2 : a firework that explodes with a loud report

Etymology: Middle French, from peter to break wind, from pet expulsion of intestinal gas, from Latin peditum, from neuter of peditus, past participle of pedere to break wind; akin to Greek bdein to break wind

Thus, to be "hoist by one's own petard" is to be blown up into the air by one's own bomb: a vivid image from the Middle Ages that applies today too.

And the etymology's valuable: you never know when it'll be handy to have the Latin for "to break wind" in your vocabulary.

(thanks, m-w.com)