This is one of the Sindarin names, used only by the Dúnedain, for one of the months of the year.

All of the months are here named: Narwain, Nínui, Gwaeron, Gwirith, Lothron, Nórui, Cerveth, Urui, Ivanneth, Narbeleth, Hithui, Girithron.
Encyclopedia entry originally written by anduril269