As you know, airports are given three-letter codes. Los Angeles International has LAX, Hartsfield-Jackson in Atlanta has ATL, John F. Kennedy in New York has JFK. Most of the time they're pretty simple, usually abbreviating the city (ATL- Atlanta, PHX- Phoenix, MKE- Milwaukee). Sometimes, though, they are not.
Those codes had to come from somewhere, though, so let's go ahead and talk that today: if not the city or the airport, what are those abbreviations actually abbreviating? (We'll stick to the United States. Canada snapped up pretty much anything available with a Y in it.)
BNA (Nashville International, Nashville, TN): Formerly known as Berry Field, so B for Berry and NA for Nashville.
CVG (Cincinnati/Northern Kentucky International, Hebron, KY): Well, it's in Kentucky, so Cincinnati couldn't get the nod, and they can't use CIN now anyway, as Carroll, Iowa took it. CVG is an abbreviation of the closest decent-sized city on the Kentucky side, Covington.
EYW (Key West International, Key West, FL): You know how radio call signs in the US start with W and K? The FCC wants those starting letters reserved for radio, so pretty much anyone in the lower 48 with a city name starting with K or W has to pick something else. Which is a problem for, among others, Key West. They ended up shifting one letter down, to make kEY West. (Not a unique solution; Norfolk International in Norfolk, VA uses ORF.)
EWR (Newark Liberty International, Newark, NJ): N's out of play too; the Navy took it.
IAH (George Bush Intercontinental, Houston, TX): HOU was out of play, and the rules say you can't have two airports with the same second and third letters within 200 nautical miles of each other. So the pre-Bush name, Houston International Airport, got the nod as a rearranged acronym. (HIA is available, though.) GBI is out, as some random airfield in the Bahamas took it, and airports usually don't change codes anyway, as they become . This will also explain the existence of, among others, BWI, DCA and IAD in the Baltimore/Washington area.
LAX (Los Angeles International, Los Angeles, CA): Airports didn't always have three-letter codes. They had two-letter codes. In the early days, that was all the airports felt they needed; many just copied the two-letter code from the National Weather Service. When it became necessary to go to three letters, some airports- Los Angeles included- just tacked an X onto the end and plowed ahead. Portland International in Oregon (PDX) and Sky Harbor in Phoenix (PHX) did the same thing.
MCI (Kansas City International, Kansas City, MO) Originally to be known as Mid-Continent International.
MCO (Orlando International, Orlando, FL): Formerly McCoy Air Force Base.
MSY (Louis Armstrong New Orleans International, New Orleans, LA): Named for Moisant Stock Yards, itself named for John Moisant, a pilot from the infancy of aviation whose plane went down in a New Orleans cemetery in 1910. The land was named for him; that's where the airport is now.
OGG (Kahului Airport, Kahului, HI): The other half of a very common flight connecting it to Honolulu (aside from island-hopping, Kahului is increasingly used as a go-between from Honolulu to the mainland). OGG is an homage to Bertram J. Hogg, who... well, someone had to make those first commercial flights between the islands.
ORD (O'Hare International, Chicago, IL): Formerly known as Orchard Field.
SNA (John Wayne Airport, Orange County, CA): The airport, while in an unincorporated ares, has its mailing address in Santa Ana.