I think it means when someone is hung by their own rope or trap?
Yes pretty much, it just means if something backfires on you. A petard was a medieval bomb used to blast through fortifications so the phrase refers to the bomb blowing up on the user. The phrase is from shakespeare.
A petard was a device used to throw huge rocks at an enemy. If a person was not careful they could easily be caught by the basket of the petard as it was fired. Hence the saying Hoist by his own petard. In the movie Lord of the Rings 3 Return of the King, the huge rock throwers used by Gandalf and the City of Ministiras are a type of Petard.
