To take credit for someone else's idea or achievement, or to upstage someone by doing what they planned to do first.
This phrase comes from 18th-century playwright John Dennis, who invented a new method for creating thunder sound effects for his play. When his play failed but another theater used his thunder technique, he complained that they had 'stolen his thunder.' The phrase caught on and generalized beyond theater.
This is one of the rare idioms where we know exactly who coined it and when! John Dennis probably never imagined his moment of theatrical frustration would become a permanent part of English. It's also fascinating that a technical innovation in sound effects gave us a metaphor for intellectual property theft.
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