not worth a hill of beans

Example: “His promises aren't worth a hill of beans.”

Origin: The "hill" is agricultural and specific, not a figure of speech. Beans are traditionally planted not in continuous rows but in small raised mounds called hills, several seeds to each — so a hill of beans is a real and countable unit of a crop, and a conspicuously minor one, representing perhaps a handful of a plant already cheap enough to serve as English's standard measure of the negligible. That precision is what makes the phrase work: it does not say something is worth nothing at all, but assigns it a real value and then names one so small as to be beneath discussion. The expression is American and nineteenth-century, and it reached a global audience through its use at the emotional climax of the 1942 film Casablanca.