- backdoor (adj.)
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[backdoor 词源字典] - 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.; compare also back-door man "a married woman's lover," black slang, early 20c.[backdoor etymology, backdoor origin, 英语词源]