A five-pound note in British slang, or a reference to science fiction author Ray Bradbury.
From British rhyming slang: Bradbury rhymes with 'five.' The term emerged in mid-20th century England, possibly influenced by the fame of author Ray Bradbury (1920-2012), though the connection is folk etymology.
This is a perfect example of how slang creates shortcuts for money—British rhyming slang invented 'Bradbury' for a fiver because it rhymes, but then the author's fame may have reinforced it. Language and culture are constantly remixing each other!
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