Use or appropriate another's idea, especially to one's advantage, as in It was Harold's idea but they stole his thunder and turned it into a massive advertising campaign without giving him credit. This idiom comes from an actual incident in which playwright and critic John Dennis (1657–1734) devised a “thunder machine” (by rattling a sheet of tin backstage) for his play, Appius and Virginia (1709), and a few days later discovered the same device being used in a performance of Macbeth, whereupon he declared, “They steal my thunder.”

To upstage someone; to destroy the effect of what someone does or says by doing or saying the same thing first: “The Republicans stole the Democrats' thunder by including the most popular provisions of the Democratic proposal in their own bill.”