The Collaborative International Dictionary of English v.0.48:

Backdoor \Back"door"\, a.
   Acting from behind and in concealment; backstairs; as,
   backdoor intrigues.
   [1913 Webster]

WordNet (r) 3.0 (2006):

backdoor
    n 1: an undocumented way to get access to a computer system or
         the data it contains [syn: back door, backdoor]
    2: an entrance at the rear of a building [syn: back door,
       backdoor, back entrance]
    3: 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]