'Back door' definitions:

Definition of 'back door'

From: WordNet
noun
A secret or underhand means of access (to a place or a position); "he got his job through the back door" [syn: back door, backdoor]
noun
An undocumented way to get access to a computer system or the data it contains [syn: back door, backdoor]
noun
An entrance at the rear of a building [syn: back door, backdoor, back entrance]

Definition of 'Back door'

From: GCIDE
  • Back door \Back" door"\ A door in the back part of a building; hence, an indirect way. --Atterbury. [1913 Webster]