1703, in reference to the language, from the name of a native Algonquian people who called themselves Inoca (1725), also written Ilinouek, Old Ottawa for "ordinary speaker." The modern form represents a 17c. French spelling, pronounced "ilinwe" at that time. The U.S. territory was created 1809, admitted as a state 1818. Related: Illinoisan (1836), which seems to be the usual form; Illinoian is used in geology to refer to one of the Pleistocene ice ages in North America (1896) and earlier it was a newspaper name (1838) and a steamboat (1837). Illinoisian (adj.) was used in England in 1818.

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