A person who lives in or is from Chicago, Illinois.
From 'Chicago' (from Potawatomi 'shikaakwa' meaning wild onion or skunk) plus the suffix '-an' (inhabitant of). The suffix '-an' is a standard way to form demonyms in English.
Chicago got its name from wild onions, so technically a Chicagoan's city is named after... a vegetable! It's the same root that appears in 'chibol' (an old word for onion), showing how indigenous words for plants shaped North American city names.
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