Neptune

The proper noun is derived from Middle English Neptune, Neptunus (“Roman god”), from Old English Neptune (“Roman god”), borrowed from Latin Neptūnus (“Roman god”), probably from Proto-Italic *neptūnos. The further etymology is uncertain; the following derivations have been suggested:

From Proto-Indo-European *neptu- (“moist substance”), possibly related to Proto-Indo-European *nébʰos (“cloud; mist, moisture”), from *nebʰ- (“to become cloudy; to become damp”).

From Proto-Indo-European *népōts (“descendant; grandson; (possibly) nephew”), in the sense of a descendant of water.

From Latin Nepe, Nepet, Nepete (now the town of Nepi in Viterbo, Italy), known for its mineral springs and in a district where a cult of Neptune (and his Etruscan counterpart 𐌍𐌄𐌈𐌖𐌍𐌔 (neθuns, “Nethuns”)) existed, from Etruscan 𐌍𐌄𐌐𐌄𐌕𐌄 (nepete), possibly from a pre-Indo-European word meaning “damp, wide valley; plain” and related to Ancient Greek νᾰ́πη (nắpē, “dell, glen, woodland vale; chasm”).

Proper noun sense 2.2 (“eighth and furthest-known planet in the Solar System”) is a semantic loan from French Neptune, coined by the French astronomer and mathematician Urbain Le Verrier (1811–1877), who predicted the existence and position of the planet through mathematical calculations.

The noun is derived from the proper noun.