What is a backdoor man?

A clandestine lover who must sneak in and out of the back door to keep things on the down low. This from blues lyrics. A song "Back Door Man" was written by Willie Dixon and done by Howlin' Wolf in the early 60's. The Doors covered the tune later in the 60's, and this is arguably the most know version, though it was covered by a number of other groups and individuals after that.