To steal someone's thunder is to take the attention meant for their moment. He stole her thunder, do not steal my thunder, stole the thunder entirely.
The story behind it is exact: a critic invented a way of making stage thunder, saw his play fail, and then heard his own effect used in another production at the same theatre. To steal someone's thunder is one of the few idioms whose origin is documented.
1phrasesteal someone's thunderTo take the attention that was meant for someone else's moment.
“It's rude to steal someone's thunder by announcing your own news at their party.”
steal someone's thunder is a taking.
Also accepted in games: steal your thunder
From the critic John Dennis, whose stage thunder effect was used in another play at the same theatre in 1704.
Based on a 1704 quote of John Dennis, a literary critic and playwright, who invented a novel method for creating a sound effect for thunder. The play in which he first introduced this method flopped. When the next play shown in the theatre, Macbeth, used his technique for simulating thunder, Dennis was reported in the press as jumping up and crying "How these rascals use me! They will not have my play, yet steal my thunder."
Etymology from Wiktionary, CC BY-SA
Practised in Idioms