What is the origin of the idiom "between you and me" and when to use it?

The idiom "between you and me" originated from the notion of confidentiality and trust between two individuals. It implies a shared secret or information that is meant to be kept private and not shared with others. The phrase is used to emphasize the exclusivity of the communication, suggesting that the information is shared only between the speaker and the listener, creating a sense of intimacy or confidentiality.