1) British jazz musician 2) Slang for the anus 
A back door is a feature programmers often build into programs to allow special privileges normally denied to users of the program. Often programmers build back doors so they can fix bugs. If hackers or others learn about a back door, the feature may pose a security risk. This is also called a trap door.
• A door in the back part of a building; hence, an indirect way.
The rear of the hole.
An entry point to a program or a system that is hidden or disguised, often created by the software's author for maintenance. A certain sequence of control characters permits access to the system manager account. If the back door becomes known, unauthorized users (or malicious software) can gain entry and cause damage.A key sequence which allows access to the program for maintenance. Only a programmer or technician would use such a sequence.
Back' door' A door in the back part of a building; hence, an indirect way. Atterbury. 
In computing, a back door is a hole in the security of a system deliberately left in place by designers or maintainers. The motivation for this 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. Historically,...Back door is slang for the anus.
[n] - an entrance at the rear of a building 2. [n] - a secret or underhand means of access (to a place or a position)
backdoor noun an undocumented way to get access to a computer system or the data it contains
When a ball is holed by going around the lip of the cup and dropping in from the back end.