A euphemistic or modified form of an exclamation or expression (often bowdlerized from a stronger word).
British colloquial English, likely a euphemistic alteration or minced oath from a stronger exclamation, possibly related to 'billy' used in oaths.
This is a fascinating example of how English speakers invent new words to express strong emotions while staying polite in company—the vagueness is part of the charm!
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