What is a Petard?

Many are using the cliche, "hoisted upon his own petard", without knowing what is a petard is.   A petard was an Elizabethan era devise to blow up barricades or gates. Shakespeare added the hoist as a bit of a joke, in Hamlet Act III. The Melancholy Dane would have his assassins hoisted (or blown up) by their own petard....."For 'tis sport to have the engineer/ Hoist with his own petard …"