Lefnui (also spelt Levnui) is a Sindarin name. The name appears to mean "fifth".
Sindarin
lefnui
place name. Fifth
lefnui
ordinal. fifth
lefnui
ordinal. fifth
levnui
ordinal. fifth
lefnui
Lefnui
lefnui
fifth
lefnui
lefnui
fifth
lefnui
lefnui
fifth
lemmui
ordinal. fifth
A river in Gondor, the fifth in the region of Lebennin (LotR/750), and so translated “Fifth” (VT42/14). It is simply lefnui “fifth” uses as a name and was thus pronounced [levnui], but Tolkien preferred the spelling Lefnui because “v” rarely appears before consonants in English (VT42/14).
Conceptual Development: When first mentioned in maps for the Lord of the Rings drafts from the 1940s, this name was first given as N. Lhefned >> Lhefneg (WR/437, TI/312), apparently earlier Noldorin words for “fifth”.