A great wave driven by earthquake or eruption, from Japanese tsunami: tsu, harbor, plus nami, wave, the wave that arrives in the harbor.
Documented
The harbor wave
Documented: tsu, harbor, port, plus nami, wave. Japanese used it for centuries for the great harbor-entering waves; English borrowed it after the 1896 Meiji Sanriku disaster and fully after 1946, displacing the misleading tidal wave. The term's scientific adoption is recent enough to be dated, twentieth-century seismology standardized it.
Assessment: Documented: the compound's transparency and the borrowing timeline. Tidal wave is the documented error it replaced.
Stories
The destination logic
The lexical insight: the wave is invisible at sea, a long shallow swell, and becomes a monster only in shallow water, at harbors. Japanese named the phenomenon by its moment of revelation, not its physics, a word built from the victim's vantage point. Seismology kept the name and the vantage point came with it.
Assessment: Stories: the word is a witness statement, not a description. The harbor is in the noun.
Interesting claims
The sent wave
The folklore stratum: older Japanese held tsunami to be sent, by angry sea deities or the great catfish Namazu thrashing beneath the islands, and the word's neutrality is modern. The scientific age kept the harbor word and dropped the sender, but the question of why the wave comes never left the language's shoreline.
Assessment: Interesting: Namazu the catfish is documented folklore with its own prints and talismans. The word is secular; the shelf it sits on is not.
Sources and further reading
Japanese tsunami: tsu, harbor, plus nami, wave (documented compound)
English borrowing after the 1896 Meiji Sanriku tsunami; scientific standardization in twentieth-century seismology, displacing tidal wave
Namazu the earthquake catfish, documented Edo-period folklore with prints and talismans (the folklore stratum)
Worth asking
Why did the world adopt a word that names the wave by its destination?