Definition - informer, stool pigeon, rat
The earliest uses of cheese-eater were boringly literal, referring to an eater of cheese. Occasionally we find the word used in a figurative manner, as in a 1653 translation of Rabelais' Gargantua and Pantagruel:
But in that he chargeth the Defendant, that he was a botcher, cheese-eater, and trimmer of mans flesh imbalmed, which in the arsiversie swagfall tumble was not found true, as by the Defendant was very well discussed.
The anonymously written The History of the Republick of Holland (1705) avers that the Old Hollanders (who were, according to the author, "despised by their Neighbours, by reason of their Boorish Manners") were called cheese eaters, as well as milk-sops and butter boxes, but we have little additional lexical evidence to support this contention.
The sense of cheese-eater we are concerned with appears to have originated in the United States in the first half of the 20th century.
Ross charged that the "cheese eaters" were usually harassed, threatened and beaten up on the "Q.T." after they ran to the Chinese guards with tales.
— Courier (Pittsburgh, PA), 15 Aug. 1953