Siouan languages
In the 19th century, Robert Latham suggested that the Siouan languages are related to the Caddoan and Iroquoian languages. In 1931, Louis Allen presented the first list of systematic correspondences between a set of 25 lexical items in Siouan and Iroquoian. In the 1960s and 1970s, Wallace Chafe further explored the link between Siouan and Caddoan languages. In the 1990s, Marianne Mithun compared the morphology and syntax of all the three families. At present, this Macro-Siouan hypothesis is not considered proved, and the similarities between the three families may instead be due to their protolanguages having been part of a sprachbund.
Mandan
The English name "Mandan" is derived from the French-Canadian explorer Pierre Gaultier, Sieur de la Verendrye, who in 1738 heard it as "Mantannes" from his Assiniboine guides, which call the Mandan "Mayádąna". He had previously heard the earth lodge peoples referred to by the Cree as "Ouachipouennes", "the Sioux who go underground". The Assiniboine are Siouan speakers. Nearby Siouan speakers had exonyms similar to "Mantannes" in their languages, for instance, Teton "Miwáthaŋni" or "Miwátąni", Yanktonai "Miwátani", Yankton "Mawátani" or "Mąwátanį", Dakota "Mawátąna" or "Mawátadą", etc.