It's crackers to slip a rozzer the dropsy in snide.

This line is a piece of comic criminal slang associated with Margery Allingham’s Albert Campion novels, where she often peppers dialogue with underworld argot to evoke 1920s–30s London’s petty-crime milieu. The sentence strings together period slang—“rozzer” (policeman), “dropsy” (a bribe), and “snide” (counterfeit money)—to produce a deliberately opaque, humorous effect. It is commonly cited as an example of Allingham’s ear for demotic speech and the playful linguistic texture of Golden Age detective fiction, in which class, profession, and subculture are frequently signaled through dialect and jargon.

The remark means, roughly, that it’s foolish (“crackers”) to try to bribe a police officer (“rozzer”) with counterfeit money (“snide”). Beyond the literal sense, the line showcases how slang can function as social code: it marks the speaker as streetwise (or affectingly so) and turns a moral/practical warning into a joke. In detective fiction, such language also underscores the gap between official authority and the criminal world, while inviting the reader to decode meaning from context—an echo of the genre’s broader pleasures of inference and interpretation.