English took this word from a disaster, and then mostly ignored it for about a century.
Tsunami is Japanese 津波: tsu, "harbor," and nami, "wave." The name is usually explained as a wave that shows up in the harbor. As Professor Robert Ramsey points out to NPR, though, these waves do not only strike harbors.
English speakers first met the word in 1896. On 15 June that year an earthquake wave struck the Sanriku coast of northeastern Japan and killed at least 22,000 people. Newspapers carried the word that summer, including the Washington Evening Star on 20 July 1896. In September 1896 National Geographic ran Eliza Scidmore's article "The Recent Earthquake Wave on the Coast of Japan," which uses tsunami in parentheses beside "earthquake wave." Lafcadio Hearn used it in his Atlantic Monthly story "A Living God" that December, and the Oxford English Dictionary's earliest quotation comes from Hearn's book version in 1897. Merriam-Webster and Etymonline both give 1896.
The word appeared so rarely in reports that it did not become widely known. Most English speakers kept saying "tidal wave," a phrase that originally meant high water caused by tides. By 1868 it was also used for earthquake waves, according to Etymonline, and World Wide Words says most people used it for them after Charles Lyell's Principles of Geology (1830). The label is inaccurate, because these waves have nothing to do with tides.
The change came late. Language Log notes that "tidal wave" stayed dominant until after the Second World War. World Wide Words reports that before 2004 the word was mostly known to oceanographers, seismologists and a small literary audience. The 2004 Indian Ocean earthquake then brought it into everyday British use.
Figurative use came earlier than that. Science magazine wrote of a "tsunami of comments" in the 1970s. The loanword had waited a long time for its moment.