The word "Sioux" is actually the French spelling of the Dakota word "Íŋktʰeŋ", which means "enemy." The French used the word to refer to various Native American tribes in the area, and it became the common term for them.
While "Sioux" is technically singular, it's important to be mindful of the cultural context. When referring to a specific person from one of the Sioux tribes, it's usually best to use the name of their tribe, such as Lakota, Dakota, or Nakota.