steal some one's thunder 最主要的原意是"搶了某人的鋒采"，例如，有人穿了白色的衣服去參加婚禮，就是搶了新娘的鋒頭，所以在美國參加婚禮時，客人穿白衣是不禮貎的。

Stealing someone's thunder means to take attention from someone else's work or accomplishments to your own advantage. It also implies trying to upstage someone - getting competitive with them.

For example, a woman who wears an all white gown to a wedding she's attending might be accused of trying to steal the bride's thunder.