Several months ago, a friend posted on Facebook a photo of his wife and son, with the message, “This guy … turned 26 yesterday with his beautiful mum by his side.”
What caught my attention, of course, was “mum.” That’s the equivalent in Britain and Commonwealth countries for Americans’ “mom,” as seen in this headline from a Scottish newspaper:
On Mothers’ Day, in May, I noticed a few more American “mums” on social media. Then, less than a week later, the Royal Wedding happened, and I believe I saw some references to Meghan Markle’s “mum,” though I can’t locate them now.
It was a little hard to research it further, in part because “mum” is also a common word for silent, as well as a short form of chrysanthemum. I eventually discovered that on Twitter platform Tweet Deck, I could create a column for tweets containing “mum” that were posted within a 100 kilometer radius of New York City. Who knew?
This has proved to be a gold mine, with an average of a dozen or so hits a day. For example:
From what I can tell, the American “mum” users tend to be female and on the young side. And the usage doesn’t seem to have extended to published sources. I would appreciate any additional observations on the matter.
notoneoffbritishisms.com 
In 2005 a very funny British film called ‘Keeping Mum’ was released
The word ‘Mum’ was a double entendre which would not work in AmE.
Maggie Smith was the ‘mum’ staying with her son-in-law (Rowan Atkinson) and his family, under an assumed name after she was released from a 30 year prison sentence for killing her unfaithful husband.
So the family were keeping ‘mum’ about ‘mum.
By the same token American jokes involving both meanings of the word ‘ass’ (donkey and butt/bum) do not work in Britain where we have two different words: ‘ass’ (donkey) and ‘arse’ (backside/bum)
How about Boston/eastern MA? As I’m certain it’s previously been pointed out by someone here, if not me:
In Boston and eastern MA, ‘mum’ (and ‘ma’ + for small children, ‘mummy’ and ‘mummah’) is common. Its use may very well be on the decline here but my mum is still ‘mum’.
I certainly don’t mean to question your experience, but I have lived my whole life in New England (not quite in “eastern MA,” but fairly close – central Mass. and Rhode Island) and have never encountered this before.
Having lived in Britain more than 50 years ago and having kept up with my host family, I’ve noticed something else: At some point my English goddaughter stopped referring to her mother as “mum” and has been calling her “mom” ever since. Don’t know if that’s a trend or not, so that could be checked into.
I’m English and alternate between ‘mum’ and ‘mom’. I’m from Yorkshire so there’s a lot of refs to ‘me mum’ flying about if I’m talking to others but ‘mom’ when talking to her, purely because I call out to her from upstairs a whole lot and ‘mommmm’ trips off the tongue easier when shouting than ‘muuuum’. But normally I’d use ‘mum’.
I have a middle aged friend from Oregon who has always called her mother “mum.” I think there’s a degree of intentional difference or quirkiness there. Probably true of many NooBs.
Similar to Boston, ‘mum’ is quite common in Vermont and doesn’t appear to be on the decline at all. And we all love our…ahhnts. (aunts)
Philip Larkin wrote a poem about Mums (and Dads).
I’m somewhat surprised to see that the story comes from a Scottish newspaper, since, in Scotland and the north-east of England, the word is pronounced “Mam”.
North West England as well, and also Wales.
How much of the north-west? I was aware of Cumbria, but is Lancashire “Mam”? Yorkshire is “Mum”
It used to be used in Widnes, Warrington, St Helens and Liverpool at least. It may be dying out now though. Liverpool used ma as well
Perhaps people think it’s cool using such Britishisms.
I don’t have much to add about “mum”, but I was somewhat surprised that this site didn’t pounce on the various usages in the American press of the word “waistcoat” in coverage of the recent FIFA World Cup. This article is a good example:
Note how it doesn’t offer any notes about language usage (and no mention of how the word is pronounced), and freely alternates between “waistcoat” and “vest”.
I think most Americans don’t notice the word “waistcoat” as odd when they see it written, but in my experience they often don’t understand it when they hear it spoken.
Mom is the preferred pronunciation in Birmingham UK.
During WW2, British propaganda posters exhorted caution in spreading sensitive information with the slogan “Be like Dad – keep Mum.”