Google's n-gram viewer finds nothing in the 1800s for "watch your mouth" and the few instances between 1905 and 1910 refer to someone else watching your mouth.
It seems an odd admonishment. You can't watch your own mouth; at most you can watch a reflection in a mirror, or the image captured by a camera. And that's not what it means, anyhow.
It's not a redefinition of the words; Horatio Alger Jr's heroes watched their expenditures. But at some point it became a common idiom. It appears to be at the start of WWII. A 1941 novel by Thomas Bell, Out of this Furnace, has a character saying “If I were you I'd watch your mouth. I don't have to take that kind of talk from anybody." Obviously, it must have existed in the oral language first. Life published a couple of 1941 stories about The amazing Visual Research Eye-Cam saying that when you smile, people watch your mouth. And this was soon to be the era of "Loose Lips Sink Ships".
But the usage didn't really take off until the mid-60s. I suspect that with bigots resenting advances in civil rights, many were admonishing each other to be careful not to ignite violence, but that's simply a gut feeling, not backed with any evidence whatsoever. Usage continued to climb, until 2003, when it dropped. I can't think of any reason why it would have dropped then. Did another phrase start to get substituted? Did "if you see something, say something" in the wake of the 9/11 attacks discourage discretion? It would be interesting to see if Ferguson affects the ngram charts.
It probably started as "mind your mouth", which is still used with other body parts ("mind your feet" to warn someone of a tripping hazard, or "mind your tail" as we used to tell our cat to make sure he was all the way through a door before we closed it).
I tell people I'm watching my weight, then pat my belly and add "fortunately most of it's right out here in front of me where I can keep an eye on it".
We got a lot of "watch your mouth" growing up in the Midwest. On at least one occasion, I recall having to "wash my mouth out with soap" after uttering a 4-letter expletive. What that involved was biting the end off a soap bar while my mother monitored, and holding it in my mouth while she counted to 10. Not sure what now-banned ingredients they had in soap back in the 60s, but it burned like hell and took quite a bit of fresh water to wash out the bitter taste.
Speaking of 4-letter words, one of my neighbors argued today (not for the first time) that there's really only one forbidden word in the American language, and it's the c-bomb.
I follow musicians on Twitter that are always saying nigga this and nigga that. I think there are no whites among them. I myself am snow-blindness white, but when I was about 20, my mother her great-grandfather was of the Lo tribe. They were high-priced slaves because they were literate and could translate other African languages, so they were used as straw bosses. That's why so many in our family became writers, she said. My sister didn't believe it, and told me she was kidding, but a $99 DNA test is consistent with her tale. That would make my great-grandparent mulatto, my grandmother a quadroon, my mother a octoroon, and I'd be a mustefino, a quintaroon or a hexadecaroon, depending on your linguistic tastes. Invariably, there is no way to specify mustefino on a census form, a birth certificate, or a voter registration form.
Anyhow, it turns out that the c-bomb has been around since 1300 AD, and there appears to never have been a time when it was acceptable to use the term, for the anatomy, as an extreme offensive term for a despicable human being, or for sexual intercourse.
When I was an adolescent, other guys would refer to a female dog (technically, a bitch) in oestrus as a cun. I don't find that word anywhere on Google. I don't know if they misheard the c-bomb word, or what.
I'm sure a doctoral candidate in women's studies could write a bang-up (is that the right word to use?) thesis on why male chauvinists were to blame for that word being forbidden, and it was to blame for women being downtrodden, but heaven help the first guy who asks about her topic.
deaconB said
Speaking of 4-letter words, one of my neighbors argued today (not for the first time) that there's really only one forbidden word in the American language, and it's the c-bomb.
deaconB, I assume that 'c-bomb' is actually 'cunt'. Grant has a post on this page that says any word can be spelled out on this forum.