Why is Thailand called Thailand?

Thailand was known as Siam until 1939. The name "Thailand" comes from the Thai word "thai," which means "free," and may derive from the Sanskrit word "thaila," meaning "land of the free." Siam became known as Thailand officially under King Prajadhipok (Rama VII) on June 24, 1939. The Siamese people had referred to their country as Mueang Thai prior to 1939.