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a door at the rear of a house, building, etc.
a secret, furtive, or illicit method, manner, or means.
secret; furtive; illicit; indirect.
Contemporary Examples

Walker makes it to his back door and gingerly steps into the inky night.
The Man Oswald First Tried to Kill Before JFK Bill Minutaglio, Steven L. Davis October 2, 2013

While we are looking across distant ridges at potential risks, real threats are coming through the back door.
Colorado Blazes Remind Us That National Policy on Fire Needs a Fix Stephen J. Pyne June 28, 2012

He opened the back door of the Humvee, where Mace was trying ease himself out.
Ty Carter Awarded Medal of Honor David Eisler, Jake Tapper August 30, 2013

Here, the young Royal leaves his beloved Arts Club in Mayfair via the back door at the weekend.
“Step Back; We’re all Armed, Angry and Tired” – Harry Leaves London Club Tom Sykes April 29, 2012

But when he returned home, he would always enter through the back door.
‘I Just Thought He Was Odd’: Neighbors of the Cleveland Kidnapper on What They Saw Christine Pelisek May 7, 2013

Historical Examples

I’m willin’; but I’m not goin’ around by the back door to miss that feller.
The Duke Of Chimney Butte G. W. Ogden

They saw Mrs. Grant rush from the back door, and then fall upon the ground.
Hope and Have Oliver Optic

He turned without waiting a second and passed through the back door by which he had entered.
The Man in the Twilight Ridgwell Cullum

They build every year in the big elm by the back door, and they do sing beautiful.
Hildegarde’s Holiday Laura E. Richards

But there was no back door, and Robin’s sentry had followed me to the wash-place, and stood stolidly by the door until I came out.
Caught by the Turks Francis Yeats-Brown

noun
a door at the rear or side of a building

a means of entry to a job, position, etc, that is secret, underhand, or obtained through influence
(as modifier): a backdoor way of making firms pay more

adj.

also back-door, “devious, shady, illegal,” 1640s. The notion is of business done out of public view. The noun back door in the literal sense is from 1520s, from back (adj.) + door. The association with sodomy is at least from 19c.; cf. also back-door man “a married woman’s lover,” black slang, early 20c.

noun phrase

A devious, shady, and perhaps illegal means: The US Government may be able to use a ”back door” into Microsoft Windows to infiltrate computers worldwide (1581+)
The anus; asshole (1600s+)

security
(Or “trap door”, “wormhole”). A hole in the security of a system deliberately left in place by designers or maintainers. The motivation for such holes is not always sinister; some operating systems, for example, come out of the box with privileged accounts intended for use by field service technicians or the vendor’s maintenance programmers. See also iron box, cracker, worm, logic bomb.
Historically, back doors have often lurked in systems longer than anyone expected or planned, and a few have become widely known. The infamous RTM worm of late 1988, for example, used a back door in the BSD Unix “sendmail(8)” utility.
Ken Thompson’s 1983 Turing Award lecture to the ACM revealed the existence of a back door in early Unix versions that may have qualified as the most fiendishly clever security hack of all time. The C compiler contained code that would recognise when the “login” command was being recompiled and insert some code recognizing a password chosen by Thompson, giving him entry to the system whether or not an account had been created for him.
Normally such a back door could be removed by removing it from the source code for the compiler and recompiling the compiler. But to recompile the compiler, you have to *use* the compiler – so Thompson also arranged that the compiler would *recognise when it was compiling a version of itself*, and insert into the recompiled compiler the code to insert into the recompiled “login” the code to allow Thompson entry – and, of course, the code to recognise itself and do the whole thing again the next time around! And having done this once, he was then able to recompile the compiler from the original sources; the hack perpetuated itself invisibly, leaving the back door in place and active but with no trace in the sources.
The talk that revealed this truly moby hack was published as [“Reflections on Trusting Trust”, “Communications of the ACM 27”, 8 (August 1984), pp. 761–763].
[Jargon File]
(1995-04-25)
An entry at the rear of a building, as in Deliveries are supposed to be made at the back door only. [ First half of 1500s ]
A clandestine, unauthorized, or illegal way of operating. For example, Salesmen are constantly trying to push their products by offering special gifts through the back door. This term alludes to the fact that the back door cannot be seen from the front. [ Late 1500s ]