1. An entry at the rear of a building, as in Deliveries are supposed to be made at the back door only. [First half of 1500s]
2. A clandestine, unauthorized, or illegal way of operating. For example, Salesmen are constantly trying to push their products by offering special gifts through the back door . This term alludes to the fact that the back door cannot be seen from the front. [Late 1500s]