Monk's moments: Hoist by my own petard

We are all familiar with the phrase ‘hoist by his own petard’ but I am not certain how many people know what a petard actually is. In fact it was a medieval bomb used in sieges and such like, and the phrase entered the English language in Shakespeare’s ‘Hamlet’ where it was used, as now, to describe someone who had fallen foul of their own deceit.