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- secret; furtive; illicit; indirect.
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.