Once upon a time, many, many moons ago, Southwest's 2-letter code was "OE" and at some point, it was decided that we wanted something else that better matched our airline and its personality. The obvious "SW" was already in use by another airline (either Air Nambia, or Seaboard World, check that, I think Seaboard was SB) but in any event, we went looking for any available 2-letter code that had a "W" in it. So, That's where "WN" came from, and all this took place in the 1970s, well before the 1980's book had some believing "WN" stood for "We're Nuts."

I got the above from an original employee that was here from Day-1, and was in a position to know, so that's my story and I'm sticking to it.

Internally (at least in my department, and usually in reference to an aircraft number suffix) we just say "W-N" or whiskey november, and we don't get into all the other interpretations.

As an aside, all the 2-letter aircraft registration suffixes like WN (as well as SW, LV, SA, CT, WR, JC, and those named for current/former higher-ups (including HK, CB, etc.) are in the process of going by the wayside. With the advent of the -800s coming into the fleet, and new system was needed, and that's why the -800s come with 8nnn numbers followed by a single, variable letter that has no meaning. Eventually, all -700s will eventually get similar treatment (7nnn and a random letter), and with the -500s almost gone and the -300s following, I don't think many of the -300s will see changes, if any.