Origins
Speed was born from violence.βββββββββββββββββββββββββββββββββββββββββββββββββββββββββββββββ Rapid comes from Latin rapidus β 'seizing, swift, fierce' β from rapere, meaning 'to snatch, to carry off by force'. A rapid current does not merely flow quickly; it grabs whatever enters it and carries it away. The word arrived in English with its claws still showing.
Latin rapere produced a remarkable family. Raptor β the bird of prey β is 'the seizer'. Rapture was originally the act of being seized and carried off, usually to heaven. Ravenous came through French raviner, from rapere β a ravenous person is seized by hunger. Ravish originally meant 'to seize and carry away by force'.
The most disguised descendant is surreptitious. Latin surripere meant 'to seize secretly' β sub- ('under, secretly') plus rapere ('to seize'). A surreptitious act is a hidden theft. The violence has been muted, but the grabbing hand remains.
Latin Roots
River rapids are named precisely. A rapid is a section of river where the current seizes objects β boats, logs, swimmers β and hurls them forward. The word preserves the Latin marriage of speed and force that other uses have softened.
English adopted rapid from French rapide in the 17th century, relatively late. Before rapid, English used swift and fast β both Germanic words. Rapid brought a Latin intensity: not just quick, but forceful. A rapid decline does not merely happen fast; it carries everything with it.