In southern culture, the phrase "back-door man" refers to a man having an affair with a married woman, using the back door as an exit before the husband comes home.
Plant references the Dixon song in Led Zeppelin's "Whole Lotta Love" (1969): "Shake for me girl, I want to be your back-door man." and also in "Since I've Been Loving You" (1970): "You must have one of them new fangled back door men!