From the wikipedia article:
The airport designator "SUX" has been contentious. Sioux City Mayor Craig Berenstein in 2002 described SUX as an "embarrassment" to the city. After petitioning the FAA for a changed airport identifier in 1998 and 2002, authorities found the alternatives offered - GWU, GYO, GYT, SGV and GAY - to be unappealing, and elected to stay with SUX. In October 2007, airport board member Dave Bernstein proposed embracing the identifier, saying "Let's make the best of it. I think we have the opportunity to turn it into a positive." and noting "I've got buddies that I went to college with in different cities that can't even remember their own birthdays, but they all know the Sioux City designator — SUX." The airport now sells merchandise with the words "Fly SUX".
Was it not hilarious that the FAA basically said, "We'll trade you a GAY for a SUX"?
Sometimes you win, sometimes you lose, sometimes it rains.