The historic Miami-Illinois people who are today referred to as the Moingona or Moingwena were close allies of or perhaps part of the Peoria. They were assimilated by that tribe and lost their separate identity about 1700. The name "Moingona" was probably the basis for the name of the City of Des Moines, the Des Moines River, and Des Moines County, Iowa.
Jacques Marquette documented in 1672 that the Peolualen (the modern Peoria). and the mengakonkia (Moingona) were among the Ilinoue (Illinois) tribes who all "speak the same language." Other names for them mentioned in 1672-73 records were "Mengakoukia," and "Mangekekis."
In 1673 Marquette and Louis Jolliet left their canoes and followed a beaten path away from the river out onto the prairie to three Illinois villages within about a mile and a half of each other. Marquette identified only one of the villages at the time, the peouarea, but a later map apparently by him identified another as the Moingwena. He said of the 1673 meeting that there was "some difference in their language," but that "we easily understood each other."
Pierre François Xavier de Charlevoix, a missionary who explored the region in 1721, recorded that "le Moingona" was "an immense and magnificent Prairie, all covered with Beef and other Hoofed Animals." He italicized the term to indicate it was a geographical term and noted that "one of the tribes bears that name." Charlevoix was a professor or belles lettres, and his spelling has come to be a preferred spelling in general and scholarly discussions.
Meaning of “Mongona”
The meaning of “Moingona” has been debated; historians have espoused conflicting definitions of the term, ranging from “People by the Portage” to “Clan of the Loon” and, more controversially, “Excrement-Faced”.
Mongona as “People by the Portage”
Historic accounts suggest that Moingona was a term referring to people who lived by, or were encountered near, the portage around the Des Moines Rapids. The noted cartographer Joseph Nicollet supported this interpretation, as did the Algonquian linguist Henry Schoolcraft. Schoolcraft and Nicollet's report says that "Moingona"
|“||is a corruption of the Algonkin word Mikonang, signifying at the road;…alluding, in this instance, to the well-known road in this section of country, which they used to follow as a communication between the head of the lower rapids and their settlement on the river that empties itself into the Mississippi, so as to avoid the rapids; and this is still the practice of the present inhabitants of the country.||”|
Moingona as “Loon Clan”
An alternative interpretation is that Moingona is derived from the Algonquian clan name "Loon"; the Miami Indian term for loon is mong’wa, and many Algonquian villages took their names from tribal clans. 
Moingona as “Excrement-Faced”
Another theory is that the root of the expression is not mi8 but m8i meaning "filth" or "excrement," and the expression means "excrement face." In this theory, the name 'Moingona', or, especially in its older French spelling, 'Moinguena', is from Illinois mooyiinkweena 'one who has shit on his face'. This etymology is supported by Gravier's word 'm8ing8eta', which he translates as "visage plein d'ordure, metaphor sale, vilain. injure". This verb, phonetically mooyiinkweeta, morphologically consists of mooy- 'shit', -iinkwee- 'face', and the third person singular intransitive suffix -ta, for a meaning "he who has shit on his face". The form 'Moinguena', phonetically mooyiinkweena, is the same verb but with the independent indefinite subject ending -na, for a more precise meaning 'one who has shit on his face'. The spelling 'Moinguena' is exactly how the French spelling of the time would render the Illinois verb mooyiinkweena. Perhaps this name arose as an insult given to the Moinguena by some neighboring tribe, as thus it is not known what the Moinguena called themselves. 
- Vogel, Virgil (1983) Iowa Place Names of Indian Origin University of Iowa Press, Iowa City.
- Thwaites, R. G, ed. (1899). The Jesuit relations, 1672-73, p. 40, 42 (French), p. 41, 43 (English).
- Perrot, N. (1864). Mémoire sur les moeurs, coustumes et relligion des sauvages de l'Amerique Septentrionale 261.
- Thwaites, R.G., ed. (1900). [Thwaites, R.G., ed. (1900). The Jesuit relations…1610-1791.
- Thwaites, R.G., ed. (1900). The Jesuit relations…1610-1791, 125-126.
- Thwaites, R. G, ed. (1900). The Jesuit relations, 1610-1791, 100.
- deCharlevoix, P-F-X. (1744). Histoire et description générale de la Nouvelle France avec le journal historique d'un voyage fait par ordre du roi dans l'Amérique septentrionale, 144.
- Nicollet, J.N. (1845). Report intended to illustrate a map of the hydrographical basin of the upper Mississippi River, 22.
- Challender, M. (2003, September 14). Is `Des Moines' just some dirty joke? Des Moines Register.
- Costa, David J. 2000. Miami-Illinois Tribe Names. In John Nichols, ed., Papers of the Thirty-first Algonquian Conference 30-53. Winnipeg: University of Manitoba.