The Popish plot. In Dryden's satire of Absalom and Achitophel, Charles II. is called David, the royalists are called the Jews, and the Papists Gentiles or Ethnoi, whence “Ethnic plot” means the Gentile or Popish plot.
“Saw with disdain an Ethnic plot begun ... `Gainst form and order they their power employ, Nothing to build, and all things to destroy.”
Part i. 518, 532-3.
Source: Dictionary of Phrase and Fable, E. Cobham Brewer, 1894