Behind Closed Doors means: In secret; Away from observers.
Example of use: "Because of all the sensitive information, they held the meeting behind closed doors."
No one can say for certain who first used the term “behind closed doors”. What is known is that it is closely related to the meaning of the word “secret”, which comes from the late 14th century Latin secretus, a term which means set apart, hidden, or withdrawn. While the exact origin of the idiom “behind closed doors” isn’t known, it is most likely an ancient saying.