Complex sentence: The word "Kalmyk" means'those who remained'--origin is unknown but this name was known centuries before a large part of Kalmyks moved back from Volga River to Dzhungaria in the 18th century.
Split into simpler sentences: The word "Kalmyk" means'those who remained'. Its origin is unknown but this name was known centuries before a large part of Kalmyks moved back from Volga River to Dzhungaria in the 18th century.