Here's a breakdown:
* Origin: The expression comes from the 17th-century play "The Tempest" by William Shakespeare. In the play, a character uses a special sound effect of thunder to create a dramatic effect. Another character, however, steals the thunder by using the same sound effect himself, thus diminishing the impact of the original character's use of it.
* Meaning: The expression has evolved to refer to any situation where someone else takes away the attention or credit for something that someone else was planning or hoping to achieve.
* Example: "My colleague was planning to announce the new project at the meeting, but the CEO stole his thunder by mentioning it in his opening speech."
In short, it means to overshadow someone else's achievement.