The phrase "cross my heart and hope to die" is a promise or oath that is often used by children to show sincerity or to emphasize the importance of their words. The phrase is typically followed by a statement or a promise that the speaker intends to keep.

The origins of this phrase are not entirely clear, but it is believed to have originated in the 19th century. The phrase may have been influenced by the Christian practice of making the sign of the cross over one's heart as a symbol of faith and sincerity.

The phrase has been used in popular culture in various ways. In the 1990s, the phrase was used in the movie "The Sandlot" to emphasize the importance of keeping a secret. In the TV show "Friends," the character Chandler uses the phrase to express his sincerity to his girlfriend.

However, the phrase has also been used in a darker context. In the song "Welcome to My Dark Side" by Kelly Clarkson, the phrase is used to suggest that the speaker is not trustworthy and may have ulterior motives. The phrase is also used in horror movies and novels to suggest that a character is making a promise that they do not intend to keep.

In summary, the phrase "cross my heart and hope to die" is a promise or oath that is often used to emphasize sincerity or the importance of keeping a promise. While it is typically used in a positive context, it can also be used in a darker context to suggest that the speaker is not trustworthy.