Yes, it's an idiom that means to take away the importance of one person's activity to make another person's activity seem more important. John was bragging about how successful he had been [ in such and such ] but then Jack stole his thunder by announcing that he (John) would no longer be working there.
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Anonymous to steal the thunderIf you're telling a joke and someone in the group butts in and tells the punch line before you do, he's stolen your thunder.