also back-door, "devious, shady, illegal," 1640s. The notion is of business done out of public view. The noun back door in the literal sense is from 1520s, from back (adj.) + door. The association with sodomy is at least from 19c.; cf. also back-door man "a married woman's lover," black slang, early 20c.
Dishonest; dubious •Earlier senses were ''secret, clandestine'' and ''illegitimate, bastard'': a sleazy little back-door business (1900s+)