Stephenson:Neal:The Confusion:84:Bonanza (Neal Stephenson)

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Literally Bonanza means fair weather or prosperity in Spanish. This alone is sufficient to explain how it came by its English meaning. I speculate---with no evidence at all---that the town of Bonanza on the Guadalquivir might have something to do with the sense of the word that means a hugely productive mine or other great source of lucre.