behind the door 

someone is "behind the door" when they only have a simple understanding of a situation, a person is ignorant or oblivious to the full meaning of a situation or is naive. Often used when talking about sexual meanings or situations.

"I don't think he understands"
"Yeh, he's a bit behind the door"

behind the door by Paul Barry Knowles May 28, 2008

Behind the door 

Being refered to as behind The door generally means a person is a bit slow or in the dark about a certain situation, they only click on to a joke days later

Kane didn’t understand much he was definitely behind the door

Behind the door by Sheeeeshkebab903 May 7, 2018

pay behind the door

to have an casual intimate encounter, usually in lieu of a favor

He suggested she pay behind the door for fixing her computer.

pay behind the door by scottishsong September 8, 2010

Lifetime of Silence, Behind the Green Door 

This is a military patch originating from the 504th flight test squadron, operating out of Groom Dry Lake, and Los Alamos AFB. Most of these projects are classified, and bearers of this patch are prohibited from discussing their occupation, hence, "A Lifetime of Silence". In a 1930's novel and a 1980's pornographic film, they both have green doors that a character cannot gain access to. In addition, the military, and most banks, use green vaults. So, the Green Door could be referencing that there are secrets that the public dosen't have access too. The true meaning of this patch is currently classified, and most likely will not be declassified in our lifetimes.

A lifetime of silence, behind the green door is the motto of the 504th Flight Test Squadron...

Lifetime of Silence, Behind the Green Door by AFC. Stanoff October 22, 2012