Lossarnach was a region and fiefdom in southern Gondor.
At the end of the Third Age, its lord was the old Forlong the Fat, who led two hundred men to the aide of the city. Forlong was killed in the Battle of the Pelennor Fields, but many of his men survived and accompanied Aragorn Elessar on his way to the Black Gate, even though most of them were farmers.
Lossarnach is glossed as "flowery Arnach". While Tolkien seemed to be convinced that the second element, Arnach, was a Pre-Númenórean word, the first element caused more problems: since loss means "snow" in Sindarin it is unclear why it "was prefixed to Arnach", although it likely derived from likeness between loss and Sindarin loth ("flower").