"Sunny Jim" was a cartoon character used to advertize a breakfast cereal, called "Force", many years ago. (Something like "Over the roofs jumps Sunny Jim, FORCE is the food that nourishes him" -- hence Andy Partridge's line "Sunny Jim couldn't jump it".)
Nowadays Sunny Jim is used as a slightly patronizing term for a young man (e.g., "Don't get lippy with me Sunny Jim"), and is usually followed by a punch in the face.
source: the Chalkhills FAQ
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