NeptuneyoudaoicibaDictYouDict
late 14c., "god of the sea," from Latin Neptunus, son of Saturn, brother of Jupiter, the Roman god of the sea (later identified with Greek Poseidon), probably from PIE root *nebh- "cloud" (source of Latin nebula "fog, mist, cloud;" see nebula), via a sense of "moist, wet." The planet so named was discovered by Galle in 1846. Until the identification of Pluto in 1930, it was the most distant planet known.
Neptunian (adj.)youdaoicibaDictYouDict
1650s as "pertaining to the god Neptune;" 1794 in the geological sense, referring to actions of water, from Neptune + -ian. Usually opposed to volcanic or plutonic. As a noun meaning "inhabitant of the planet Neptune" it is recorded from 1870.
neptunium (n.)youdaoicibaDictYouDict
1941, from Neptune + element ending -ium. Named for its relative position in the periodic table, next after Uranium, as the planet Neptune is one beyond Uranus. See also plutonium.