bonanza

English borrowed bonanza from Spanish bonanza, meaning dead calm, fair weather, good luck, or a rich lode. That Spanish word came from Vulgar Latin *bonacia, a term for a lull or dead calm, which itself descended from Latin malacia, meaning calm sea.

The shift from calm sea to good fortune happened because Vulgar Latin speakers misread malacia's initial mal- as coming from Latin malus, meaning bad. Since a calm sea felt like the opposite of bad, the word was reshaped under the influence of bonus, meaning good, turning a term for stillness into one for luck and abundance.

Bonanza's remade form was influenced by Latin bonus, meaning good — the same root that gives English words like bonus itself.