To most Americans, the word hoosier means "person from Indiana." Indianans proudly call themselves hoosiers. In my native St. Louis, however, the meaning of the term is rather different: it's a very insulting term meaning, basically, "hick" or "redneck." Oddly, the hoosier-as-hick usage seems to be totally specific to the St. Louis area.
The Wikipedia article on "hoosier" specifically mentions the strange St. Louis usage: "In the St. Louis area of Missouri and Illinois, a hoosier may be someone who is lower-class and white (like white trash and redneck)." The Urban Dictionary lists 20 definitions of hoosier, 10 of which focus on and explicitly mention St. Louis. [One reader gives an example of how to use it: "I got cut off on Highway 40 this morning by some fucking Hoosier driving a Hemi covered in American flags and "W" 2004 stickers."] In a long essay exploring the origins of the term hoosier, Jeffrey Graf of the University of Indiana devotes two paragraphs to the St. Louis usage.
So what is a "hoosier"? And why do St. Louisans use it in a way that no one else does? Well, no one really knows. Some think that it originally meant country bumpkin and that the negative usage faded almost everywhere but St. Louis. Others think it was originally a greeting--"who's there?"--that got slurred over time. The links above list other theories.