köken: Probably from Proto-Italic *neptūnos, from Proto-Indo-European *nébʰos (“cloud, moisture, mist”), but this is disputed; see Neptune.
Etimoloji
Appears to be borrowed from Chinese 海王星 (Hǎiwángxīng), applying the expected Japanese 音読み (“on'yomi”) for the characters. The planet itself was discovered in 1846 by astronomers in Europe, so prior to that point, the planet had no name in any language. Following the convention at the time, it was ultimately named for the Roman god, Neptune. After this, the term was probably first borrowed into…