means a section of a river where the water flows fast and turbulently over rocks, often with white foam and a steep gradient

from the plural of 'rapid,' from the Latin 'rapidus' meaning 'seizing, swift, tearing away,' built on 'rapere,' to seize or snatch — the same grabby root that gives us 'rapture' and 'rapacious.' The water, in other words, was named for its hurry: a river caught in the act of grabbing at everything in its path.