They are the affectation of affectation.

Fielding’s line is a compact piece of satiric dismissal: it describes some group or practice not merely as artificial (“affectation”) but as a second-order, self-conscious performance of artificiality—posturing about posturing. The phrase suggests a social world in which sincerity has been replaced by mannerism, and even mannerism has become a fashionable pose in its own right. In Fielding’s comic-satirical moral universe, such layered pretence is especially contemptible because it signals both vanity and emptiness: there is no underlying substance, only the display of display. The sting of the wording lies in its recursive structure, which mimics the hollowness it condemns.