Q From Sharon Warne: Why do we Americans call bathrooms johns?
A An interesting question. I wish we had a good answer. All I can do is provide you with some pointers.
It was at one time common to address any man whose name one didn’t know as John, particularly a servant (much like a Glaswegian will use Jimmy today, and others will use Jack). There are dozens of expressions recorded that contain John as a generalised name, such as John Thomson’s Man for an uxorious husband, Johnnny Foreigner for anyone from outside Britain, John Trot for a bumpkin, John Tuck for a Chinese mandarin, and John Barleycorn, the personification of the barley from which whisky is made.
And there is John Thomas as a slang term for the penis; this is known in print since the 1870s, but since it was rude it had probably been in circulation for a time before anyone wrote it down (there’s a story that there was a real John Thomas about 1400 who was extremely well-endowed, but it seems to be just a legend; it’s more probable that it derives from the use of the same phrase as a generalised term for a servant). Associated words are the comparatively recent johnny as one of a myriad of slang terms for a condom, and john for a prostitute’s client.
An even older term for a privy, jakes, is sometimes said to be derived from the Elizabethan Sir John Harrington (who invented the first water closet). But it’s actually older, though probably derived from Jacques.
The first recorded use of john for a privy is in the college customs of Harvard College about 1734-5, which were written down by one Richard Waldron of the class of 1738. Rule 20 states “No freshman shall mingo against the College wall or go into the fellows’ cuz john”. “Cuz john” here is short for “cousin John”. I would guess there was a euphemistic phrase of excuse lurking here, along the lines of “I’ll just go and talk to Cousin John”.
[That word mingo, by the way, means “to piss”, and is related to other obsolete words with the same sense, such as minge and miction. It comes from the Latin mingere, ‘to urinate’, hence our micturition. It’s obscure enough that even the OED doesn’t gloss it, though it turns up twice in citations for other words. It was also an eighteenth-century term for a chamber-pot.]
And as a trans-cultural final note, john is not used in Britspeak; indeed, we don’t talk about bathrooms in this context either, since we prefer toilet, or loo, or sometimes lavatory.