The Oatmeal asks this question.

We have discussed these euphemisms a few times before, for, instance, in this thread.

He asks "Why not simply restroom, toilet, or bathroom? My guess is that the airline is British or Commonwealth-owned, or the plane is built outside the USA.

Over here, "restroom" and "bathroom" are not used in this context, and "toilet" used to be (and may still be with some) considered rather infra dig. In any case, unless you are a millionaire with your own private jet, you won't find a bath on board a plane!

Kalleh and any other frequent flyers, do you recall what's on the doors of the facilities of planes you've used?

In the U.S. we do tend to use restroom or bathroom, but it is different in airports, and I think it's because we're trying to be more "global."

I do remember lavatory being used in my elementary school (third grade, I think). I always thought they were saying labatory.

Our doors generally just had a half-moon carved in them.

You have doors? Lavatory is silly since it doesn't describe whether the lava is aa, pahoihoi, pyroclastic, or what. And if you sit where the lava comes from, you're on the hot seat fer sure!

Anyhow, on the airlines I fly on, the sign says, "baño."

A bathroom can also be referred to as a W.C., can, comfort station, commode, garderobe, gentlemen's room, head, john, ladies' room, latrine, lavatory, little boy's room, little girl's room, men's room, outhouse, potty, powder room, privy, restroom, throne, washroom, water closet, women's room.

Or used, in slang: as

altar room, ass-wiper, bean-jacks, bung fodder, flusher, gents, the, throne, throttle pit, toilet service, toilet servitude, toilet sex, toilet training, Tijuana racetrack, retreat to one’s sanctum sanctorium, retreat to the holy of holies, perform the work of nature, pit stop, go tap a kidney, go to Egypt, take an Irish shave, talk on the great white telephone, talk to god on the big white telephone, pray at the porcelain altar, pray to the porcelain god, pray to the porcelain goddess, pray to the porcelain gods, pray to the porcelain princess, visit the chamber of commerce, visit the loo, visit the potty, waterloo, worship at the porcelain altar, go and sing Sweet Violets, go feed the goldfish, go into one’s private office, go into retreat, go to the bank, go to the can, go to the crapper, go to the dunnee, heed nature’s call, hug the porcelain wishing well, hug the throne, one and two, pay a visit, pay a visit to the old soldier’s home, butt tape, cash a check, biffie, bumf, bumph, check the plumbing, comfort room, comfort station, cottager, drive the big bus, feed the dog, small room, smallest room, bog queen, bog-house, make an offering to the porcelain god, make food offerings to the china gods, make a coke stop, make a phone call, shouse, sing psychedelic praises to the depths of the china bowl, answer a call to the warden’s office, answer nature’s call, answer the call of nature, kiss the porcelain god, kneel before the porcelain throne, loo, Johnny Queen, johns, ass-wipe, bog, butt-wipe, peecam, washroom faggot, privy queen, tea room queen, tearoom queen, T-room queen, stool pigeon, throne queen, crapper, latrine, jane, back room, anaclisis, anaclitism, sit on the throne, make a pit stop, see one’s aunt, see one’s auntie, shit-house, shithouse, shitter, bogs, bidet, biff, have Chinese singing lesson, go potty, go see, go stinky, get more ass than a toilet seat, gets more ass than a toilet seat, give the Chinaman a music lesson, casual contact, chic sale, toilet cam, toilet queen, toy-toy, TP, trots, the, visit Miss Murphy, visit Mrs. Jones, Toilet Tessie, swords, bum fodder, go to pot, worship the porcelain god, worship the porcelain goddess, worship at the altar, visit the Widow Jones, bathroom, john, closet queen, clubhouse, coprophagia, bucket

Here's something from {i}The Totally Useless AHIstory of the World[/i] by Ian Crofton:

"circa AD 79... The Roman Emperor Vespasian imposed a tax on public lavoratories. His son Titus was horrified at such an undignified measure, but Vespasian would have none of it. Thrusting a handful of coins under his son's nose he declared "Pecunia non olet' (money does not smell). In honor of the emperor, the public urinals that once graced many a street corner in French towns and cities were known as vespasiennes. The vespasiennes, however, did smell - pungently so."

Wow, proof. Where did you get all those slang words for toilet?

Yes, I wonder too. It looks like most of those words and phrases don't refer to toilet, but rather to the person using the toilet, the act of going to the toilet, one's performance there, toilet paper, etc. "Gets more ass than a toilet seat" refers to sex. Some of them I don't understand at all.