• 05.115 It’s off to the back-door.
• 05.115 To the back-door,
• 05.123 This is the way to the back-door,
• 06.030 except that Gandalf knew all about the back-door,

Given in OED as “backdoor” with the hyphenated version in the examples.

“backdoor, n.” OED Online. Oxford University Press, June 2017. Web. 5 September 2017.