'Hoosier' is a popular toponym for a person from Indiana. According to the official website of Indiana's own government, "For well over a century and a half the people of Indiana have been called Hoosiers. It is one of the oldest of state nicknames and has had a wider acceptance than most."
The same page tells us that the term was in general use by the 1830s, and the earliest reference it gives is from a letter written in 1831. Citing its use in a poem printed in the Indiana Journal two years later, it suggests that "Apparently the poet felt that it was sufficiently familiar to be understandable to his readers."
There are several suggestions as to the origin of the term. More than one of them refers to a rendering of the words "Who's here?", used for example when someone knocks on the door of a settler's cabin. (You might think that "Who's there?" would be more appropriate, but that obviously wouldn't work as an explanation!)
© Haydn Thompson 2021