The telegraph is the origin of using an "X" to abbreviate or represent a bunch of following letters.

Other examples:

WX - weather
MX - maintenance
TX - transmit
RX - receive
DX - dispatch(er)
SEX - oh, nevermind.

Shouldn't that be "Just be at pax with it"? :)

^^Love that one! Let the Latin roll! BTW, DX is also "distant" for ham radio operators (which also came from telegraphy).

LAS in Las Vegas is for LAST, as in Last hurrah !

I think the abbreviation (probably related to telegram use) was originally used by cruise and steamship companies. Pass. couldn't be used as everyone needed a boarding pass. So, pax came about. I was travel agent for a long time and every cruise line uses pax.

Simply to avoid a poorly written 5 or S from being confused, many people were illiterate in the railroad days when this stuff became standard.

Could it have anything to do with not causing confusion with the french "ne...pas"?

Interesting.
I had always assumed it was the plural, therefore easier to use as PAX for all the passengers. And it went into singular use as being "already in place" and therefore simple.

What is #PaxEx? I see this on Wandering Aramean's Twitter all the time.

Julius Sumner Miller said "why is it so!"

this journal from 1946 has a reference to it

http://i.stack.imgur.com/a6fDF.png

As I understand it, LAX (and a slew of others) date from a very old shift from two-letter codes to three-letter codes back in the 40s. In a number of cases, I believe the airports switched codes (usually with a new airport) but in others the codes stuck. PDX, with a similar code situation, dates from 1940. A number of the airports with similarly "logical" codes (MIA, TPA, etc.) are also pretty old (Miami predates WW2, Tampa back to the 20s...and I strongly suspect that DCA would have simply been DC back in the 40s).

It's a standard travel industry abbreviation going back to the days of telex (and maybe before that to telegram). Not sure anyone knows anymore why it's pax and not pas, but given it went back to telex and printing tech wasn't quite what it is today, your suggestion about confusion between S and 5 may be correct.

As you mention it, what is "telex" short for? ;)

Well, I'm not old enough to know first hand ;-) and have seen various versions. The most common one I've seen is Teleprinter Exchange, but have also seen Teletype and Teletypewriter. Of course, Telex itself can be abbreviated to TLX in telex-speak...getting silly now.