To "steal someone's thunder" is to steal their ideas or work, or to prevent them from getting acclaim from their work by using it publicly before they do. The phrase first appeared in print in 1909, but was coined two centuries earlier by John Dennis, a playwright and critic.
In 1709, Dennis simulated thunder using a mustard bowl. He was producing a dramatic work entitled Appius and Virginia at Drury Lane Theatre. Unfortunately, his work had a very short run; presumably not because of any lack of special effects. In fact, the theatre-manager was rather taken with the device, and used it soon after in a production of Macbeth.
John Dennis attended the play. The inventor recognised his creation in action, and apparently said...
"See how the rascals use me. They will not let my play run, and yet they steal my thunder!"
Jeffrey Kacirk's Forgotten English: A 366-Day Calendar of Vanishing Vocabulary and Folklore 2004
Take Our Word For It: http://www.takeourword.com/TOW111/page2.html
The Wordsworth Dictionary of Idioms