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"
by Paul Barry Knowles May 28, 2008
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
by Sheeeeshkebab903 February 18, 2018
to have an casual intimate encounter, usually in lieu of a favor
He suggested she pay behind the door for fixing her computer.
by scottishsong September 8, 2010
It is where a tremendous amount of hypocracy starts in our society.

It is a good song by CHARLIE RICH.

It is any ASS behind a door that is shut

It is a golden opportunity to commit an unspeakable SEXUAL ACT especially if you are drunk or high or a date raper.
They can criticize you about talking about little girl's but they get so drunk as what they do BEHIND CLOSED DOORS is probably the very thing they condemn where it is with many other people also the case.

As NO ONE knows what goes on BEHIND CLOSED DOORS.

Don't be such an ASS DAN as most of the time nobody can see you being an ASS BEHIND CLOSED DOORS.

That BEHIND you are after is BEHIND CLOSED DOORS so have at it as they excuse themselves because of their state.
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...
by AFC. Stanoff May 13, 2012
1. Death. "Door number one" is the coffin lid, the ground above the grave or the gravestone.

2. Severe consequence.

"You want to jump off, Jamf, or do you want what's behind door number one?"

"Do you think H likes what he found behind door #1?"
by TmmR January 23, 2008
There is another opportunity out there.

Take a chance.

Similar to there are plenty of other fish in the sea.
John: I don't like my job.

Mary: Behind every door there is another bail of hay.
by jak8012 July 24, 2018