The -las element of Nellas' name is almost certainly 'leaf' (and this fits well with the fact that she lived in the woods of Doriath). The initial nel- is, however, less clear. It perhaps represents a shortening of neldor, the word for 'beech' in the language of Doriath (so Nellas in full would mean 'beech leaf').

As an alternative, there is a Sindarin word nella related to the ringing of bells (deriving from a root nyel- for a song or sweet sound) so on this theory Nellas could be taken to mean something like 'singing leaf'. Finally, in some contexts, -las as a name ending can theoretically derive from a root meaning 'joy', so Nellas might mean something like 'joyful song'.