The etymology of Latin Neptunus is unclear and disputed. The ancient grammarian Varro derived the name from nuptus i.e. "covering" (opertio), with a more or less explicit allusion to the nuptiae, "marriage of Heaven and Earth".
Among modern scholars Paul Kretschmer proposed a derivation from IE *neptu- "moist substance". Similarly Raymond Bloch supposed it might be an adjectival form in -no from *nuptu-, meaning "he who is moist". Georges Dumézil though remarked words deriving root *nep- are not attested in IE languages other than Vedic and Avestan. He proposed an etymology that brings together Neptunus with Vedic and Avestan theonyms Apam Napat, Apam Napá and Old Irish theonym Nechtan, all meaning descendant of the waters. By using the comparative approach the Indo-Iranian, Avestan and Irish figures would show common features with the Roman historicised legends about Neptune. Dumézil thence proposed to derive the nouns from IE root *nepot-, "descendant, sister's son".
More recently, in his lectures delivered on various occasions in the 1990s, German scholar Hubert Petersmann proposed an etymology from IE rootstem *nebh- related to clouds and fogs, plus suffix -tu denoting an abstract verbal noun, and adjectival suffix -no which refers to the domain of activity of a person or his prerogatives. IE root *nebh-, having the original meaning of "damp, wet", has given Sanskrit nábhah, Hittite nepis, Latin nubs, nebula, German Nebel, Slavic nebo etc. The concept would be close to that expressed in the name of Greek god Όυράνος (Uranus), derived from IE root *h2wórso-, "to water, irrigate" and *h2worsó-, "the irrigator". This etymology would be more in accord with Varro's.
A different etymology grounded in the legendary history of Latium and Etruria was proposed by Preller and Müller-Deeke: Etruscan Nethunus, Nethuns would be an adjectival form of toponym Nepe(t), Nepete (presently Nepi), town of the ager Faliscus near Falerii. The district was traditionally connected to the cult of the god: Messapus and
Halesus, the eponymous hero of Falerii, were believed to be his own sons. Messapus led the Falisci and others to war in the Aeneid. Nepi and Falerii have been famed since antiquity for the excellent quality of the water of their springs, scattered in meadows. Nepet is considered a hydronymic toponym of pre-Indo-European origin widespread in Europe and from an appellative meaning "damp wide valley, plain", cognate with pre-Greek νάπη, "wooded valley".