A sudden surge of support or feeling among many people, or large ocean waves caused by distant storms.
From ground + swell. Originally a nautical term (17th century) for waves generated by distant weather; metaphorically used since the 19th century for sudden surges of public opinion.
A groundswell ocean wave travels thousands of miles from its birth in a distant storm before reaching shore—it's why surfers can catch perfect waves from hurricanes happening on the other side of the ocean.
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