From Middle English Neptune, Neptunus, a borrowing from Latin Neptūnus, possibly from Proto-Indo-European *nebʰ- ("damp, cloudy") (source of Ancient Greek νέφος, Sanskrit नभस्, etc.), with the sense of "wet, moist."
Puhvel, however, connects it with Proto-Italic *nepōts from Proto-Indo-European *népōts by analogy to Portūnus, cognate with Vedic Apām Napāt and Avestan Apąm Napāt, Child (cf: neve, nephew) of the Water as part of an Indo-European 'Fire in the Water' myth.
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