Hehe, is there actually a proper word? Timothy Sly once wrote an interesting article on that subject. I'll paste it here:
The eternal problem is the exasperatingly euphemistic nature of polite English discourse. Simply put, we do not have a correct term for such a place. Not one.
“Toilet” denotes the act of cleansing/washing. So do “lavatory” & “washroom”, but no mention is made of the real reason for the visit. “Bathroom” would be valid if we needed a shower or a soak in the tub. “Powder room” suggests a place exclusively reserved for adding talcum or scented corn-starch to one's skin. “Ladies' room” or “men's room” is curiously evasive about what these gender-overconfident folk actually do in there, to say nothing of the folk who identify with other categories included in the LGBTQ spectrum.
“Rest room” is even more obscure from any accurate label; one wonders who actually enters such a facility to enjoy a rest. “Water-closet” hearkens back to early days of flushing in Britain, and it is getting close, but “public convenience” could equally apply to a drinking fountain or park bench, neither of which would invite excretorial emissions in their proximity.
“Loo” dates even further back to the 17th century when contents of “chamber pots” (yet another rich topic for euphemistic debate) were flung from upper floors onto the street and it's unfortunate occupants. The cry would go out “Guardez-l'eau!” (watch out for the water), followed by the unsavoury contents being unceremoniously hurled upon passers-by. “L'eau,” became “Loo” to the local denizens, and thus it evolved into the current meaning.
Even more confusing to overseas visitors is the reference by older British people to “I'll just go and spend a penny!” This of course refers to the cost of one penny to enter the door of the stalls at that time. Men were mostly able to avoid this, representing an early form of gender discrimination.
The “bog” was a term much beloved by schoolboys, and may have reflected the squelchy nature underfoot.
The French at least are to be congratulated for one terminology in as much as “la pissotière” was nothing less than a public urinal on the sidewalk of the busy street in which the heads and feet of the usually male occupants were still visible to those passing by, but the purpose of the facility as its name suggests, was not in doubt.
