What is the origin of the idiom "gentleman's agreement" and when to use it?

The idiom "gentleman's agreement" originated from the societal norms and customs prevalent among gentlemen in the 19th and early 20th centuries. It refers to an informal and unwritten understanding or arrangement between two parties, typically men, based on trust, honor, and integrity. The phrase is now used to emphasize the reliance on mutual respect and good faith, where a handshake or verbal agreement is considered binding, without the need for legal documentation or formal contracts.