Q From Alden Harwood in the USA: My dictionary gives the origin of groundswell in terms of a deep ocean wave, which seems plausible, I suppose, notwithstanding that we do not think of the ocean floor as ground, per se. How did it get to mean increasing popular support?
A As you say, groundswell was originally a sailor’s word for a deep ocean swell, such as might be generated by a distant storm or earthquake. It’s an odd term, and I had to hunt around to find out why it was so named.
It seems that the first sense of the word ground in English was that of a place covered by water. It was used especially of the bottom of the sea, and turns up in this sense in the early epic poem Beowulf. Two nautical phrases preserve this sense: to break ground, meaning to heave the anchor clear of the bottom, and to run aground. This second phrase predates by several centuries the use of ground to mean dry land. So a groundswell was presumably one so huge that its troughs seemed to reach the bottom of the ocean.
The figurative use grew up at the start of the nineteenth century to describe some kind of political or social agitation, an obvious allusion to the effect on a ship of encountering such a swell. This example is from 1856: “The religious world was rocking still with the groundswell that followed those stormy synods”. It has since shifted meaning to refer to any up-welling or build-up of opinion in a section of the population, taking its reference from the peaks of the swell rather than from its troughs.