noun
1.
a. : a door in the back or to the rear of something, especially a habitation
b. : an entrance or approach (as to a country) regarded as at the back and usually distant or geographically opposite the main route of approach
but the richest area of all the peninsula was found … at the back door of Nome — Encyc. Americana
the back door of Egypt — Hassoldt Davis
2. : an indirect, surreptitious, underhanded, or illegal means or way
the West entered America's popular literature through the back door of humor — J.D.Hart
depends more and more on western back doors for essential war supplies — Amerasia
a junk shop with a back-door trade in hides from illegally killed cattle — H.L.Davis