back door

US: (bak′dôr′, -dōr′)

WordReference Random House Unabridged Dictionary of American English © 2025

back′ door′, 

1. a door at the rear of a house, building, etc.
2. a secret, furtive, or illicit method, manner, or means.
• 1520–30

back•door  (bak′dôr′, -dōr′), adj. 

1. secret;
   furtive;
   illicit;
   indirect.

Also, back′-door′. 

• adjective, adjectival use of back door 1605–15

Collins Concise English Dictionary © HarperCollins Publishers::

back door noun

1. a door at the rear or side of a building
2. a means of entry to a job, position, etc, that is secret, underhand, or obtained through influence
3. (as modifier): a backdoor way of making firms pay more