"Hiro" was taken from Ōe no Hiromoto, an ancestor of the Mōri family, and "Shima" was taken from Fukushima Motonaga who helped Mōri Terumoto choose the castle site.
Ōe Tokihiro, the younger son of Ōe no Hiromoto, a senior retainer of the Kamakura shogunate was granted lands in Dewa Province, and in 1238 changed his name to Nagai Tokihiro.