What is the origin of the idiom "behind closed doors" and when to use it?

The idiom "behind closed doors" originated from the literal notion of conducting activities or discussions in a private or confidential setting, away from public view. The phrase alludes to the practice of closing doors to create a barrier between the inside and outside world, ensuring privacy and discretion. It is commonly used to describe situations where important or secretive matters are being discussed or decisions are being made privately, without public scrutiny or interference, often implying that the truth or true nature of something is hidden or undisclosed.