This study presents new data on Zamucoan ethnonymy and solves an etymological problem concerning the term Ayoreo. The earliest documented Zamucoan language is Old Zamuco, spoken in the 18th century in the Jesuit missions of Chiquitos and close to present-day Ayoreo. The morphology and meaning of ethnonyms used by Zamucoan peoples in the 18th century are analyzed. Previously undocumented ethnic denominations emerge, including the endonym of the Old Zamuco-speaking people. Some Zamucoan ethnonyms are not from Old Zamuco, but from another Zamucoan variety spoken in the 18th century, which shows a plural suffix identical to that of present-day Ayoreo. While it is confirmed that Zamuco is an exonym, this article provides new evidence that the term Ayoreo was not borrowed, since a cognate is attested in Old Zamuco. Finally, although Ayoreo, “people, human beings,” is an autochthonous word, its adoption as an ethnonym is relatively recent and is due to social dynamics common to other Chacoan populations after the Chaco War.