The reasons for the dual naming of the river as Neris by the Lithuanians and Viliya (formerly Velja, meaning "big, great" in Slavic) by the Slavs are complex.

Therefore, it has been proposed that the Narač River had in fact been considered the upper reaches of Neris by the Balts in ancient times.

[4] Some linguists, however, point to the fact, that Viliya in it upper part is not "big" or "great" at all and the name cognates with Lithuanian words velnias and vėlė.

The name Neris is of Baltic origin, a cognate of the Lithuanian nerti generally meaning "to dive, swim downstream" as well as "to net, crochet".

Etymologically, the name is one of a class of hydronyms, widespread in the modern and prehistoric Baltic ranges; e.g., Lithuanian Narotis, Narasa (rivers), Narutis (lake), Old Prussian Narus, Nara near Moscow.