Any bitchy girl who lives in Illinois. Could also be a variation of a bitchy girl from Iowa, or Indiana, etc. More generally used and accepted when a person who lives in or is deeply involved with another state is talking about someone from an "I" state.
Mizzou (or any other non "I" state college) student: Ugh, I don't want to go home for the holidays. I'll have to deal with those old highschool iBitches all over again.