"devious, shady, illegal," 1640s, from back + door. The notion is of business done out of public view. The association with sodomy is at least from 19c.; also back-door man "a married woman's lover," black slang, early 20c.
The Dictionary of American Slang, Fourth Edition by Barbara Ann Kipfer, PhD. and Robert L. Chapman, Ph.D. Copyright (C) 2007 by HarperCollins Publishers. Cite This Source
Idioms and Phrases with back-door
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
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