steal somebody's thunder

steal somebody’s ˈthunder

spoil somebody’s attempt to surprise or impress, by doing something first: He had planned to tell everyone about his discovery at the September meeting, but his assistant stole his thunder by talking about it beforehand.In the eighteenth century, the writer John Dennis invented a machine that made the sound of thunder for use in his new play. The play was not a success, and was taken off and replaced by another play. When Dennis went to see the other play, he was angry to hear his thunder machine being used and complained that ‘...they will not let my play run, but they steal my thunder’.
See also: steal, thunder