Where Did It Come From?
The idiom "to steal one's thunder" means to: 1) appropriate and use someone else's idea or plan, taking credit and praise away from them, or 2) upstage someone's moment of glory.
In the early 1700's, Dennis wrote a play entitled Appius and Virginia which opened at the Drury Lane Theatre in 1709. Unfortunately, it was a big flop, and it quickly closed. This wouldn't have occasioned much notice, except for one thing. At this time, theaters struggled to produce realistic sounding thunder when needed for storm scenes in their productions. Some of the methods used included rolling metal balls in troughs, rattling lead shot in bowls, or shaking sheets of metal. When putting on Appius and Virginia, however, Dennis came up with a way to make a more realistic thunder sound, improving on the bowl method by rolling metal balls in them instead of iron shot. After they cancelled his play, Drury Lane put on Macbeth which, of course, requires thunder. They decided to go with the method which Dennis had developed, since it sounded better. Guess who wasn't impressed by this? Yep. John Dennis was in the audience one night and recognized the distinctive sound of his thunder. Outraged, he leaped to his feet and shouted, "How these rascals use me! They will not have my play, yet steal my thunder!" Well, at least this time he didn't stab anybody.