The -duinë element of this name is definitely 'river', but the initial nun- is less clear. There are rare attested cases in Quenya of nun- being a derivative of numen, 'west', and as the river ran westward through Númenor, an interpretation of something like 'western river' seems appropriate. The Elvish word for 'west' ultimately derived from 'down' (that is, it was the direction of the sunset) so an alternative interpretation might be 'sunset river', though even in this case the association with the west remains relevant.