The story
From the name of a Native American village near present-day Tallahassee, Florida, transcribed in Spanish as Apalchen or Apalachen [a.paˈla.tʃɛn]. Originally the name of the Apalachee, a Muskogean people of northwestern Florida, perhaps from Apalachee abalahci "other side of the river" or Hitchiti (Muskogean) apalwahči "dwelling on one side"; compare Proto-Muskogean *apiCi (“stem, stalk”). The name was eventually used also for the tribe and for a region spreading well inland to the north. After the de Soto expedition in 1540, Spanish cartographers began to apply the name of the tribe to the mountains themselves.
Etymology from Wiktionary, CC BY-SA 4.0.