Officially, it's the Illinois Secretary of State's Office's "Inhibit List."
The list of 5,577 license plates represents combinations letters and numbers that can be dirty, offensive and sometimes pretty creative combinations that won't get stamped on your vanity plates in the state. The office keeps a running list of plates that have been requested and deemed offensive, and also proactively adds words they don't want to see on the road.
Most are obvious (think of any filthy or offensive word, and it's pretty much a sure shot it's banned), but others are, well, colorful. Check out some examples of what's not allowed on your bumper.