Plural of dynamo: electrical generators that convert mechanical motion into electrical energy, or informal for an energetic, powerful person.
From Greek dynamis 'power'. The term became popular in the 1880s-1890s when these electrical machines revolutionized power generation and distribution.
Edison's dynamos powered the first electric lights in New York City—they were called 'dynamos' because they converted the dynamic motion of turning a crank into lightning-like electrical power.
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