To steal someone's thunder is to take the attention, recognition, or impact that should rightfully belong to another person. Announcing your own engagement at a friend's wedding steals their thunder. A company that launches a competing product the day before a rival's planned launch steals their thunder. The phrase implies that the attention you received was borrowed or taken without permission.
This is one of the very few English idioms with an identified inventor and a specific documented incident. Playwright and critic John Dennis invented a machine to simulate the sound of thunder for his 1709 play Appius and Virginia at the Drury Lane Theatre. He did this by rolling metal balls in a bowl — the technique worked, but the play was a failure and was pulled from the programme.
Shortly after his play closed, Dennis attended a performance of Macbeth at the same theatre and heard his thunder-making technique being used without his knowledge or permission. His response was reportedly furious: 'Damn them! They will not let my play run, but they steal my thunder!' The story was widely repeated, and Dennis's exact phrase entered the language as a metaphor for any act of taking someone else's idea or credit.
The story is well documented and consistent across contemporary accounts. Dennis was a real figure — a playwright and influential critic who also coined the word 'critic' in its modern theatrical sense. The thunder machine story is supported by enough independent sources to be accepted by most language historians as accurate.
He used a technique of rolling or striking metal balls or sheets. Some accounts describe a resinated sheet of tin. The exact mechanism is lost, but it was effective enough that the Drury Lane theatre continued using it after his play closed — which was precisely the provocation that generated the famous outburst.