The Boston Globe has an amusing piece today about the different American and British meanings for pants: “trousers” here and “underpants” there. No sign yet of the Britishism getting any traction in these parts. If that does ever happen, it will kick the venerable expression “keep your pants on” up a few notches.
notoneoffbritishisms.com 
I’m from South London and my fiancée’s from Manchester. She uses the word ‘pants’ for trousers, I don’t. There are so many regional variations in words in the U.K. that you could travel 5 miles and one word could mean something else.
One I’ve noticed in particular on my travels is different words for bread rolls. Take a look at this facebook group for example: http://www.facebook.com/group.php?gid=6493347529. There are others arguing for the other words. In my experience, people can get very defensive about it all!
I always thought using pants for trousers was an Americanism!! Pants to me has always meant underwear for boys: Girls wear knickers, boys wear pants. My first hearing of the word pants for trousers appeared to refer to casual combats, but in general I don’t hear it used that way.
Interesting; to me, pants can mean either girls’ or boys’ underwear, but knickers are only for girls (perhaps reflective of the fact that acceptable crossdressing is one area in life where men *really* get shafted!). I’d never heard pants for trousers in the UK either, and I have a Dad from the North of England, so that’s confusing. I tend to use “trousers” and “underpants”, unless it’s very obvious that I’m talking about underpants; it can be important to make the distinction, particularly in the context of any “…realised I wasn’t wearing any pants” anecdotes.
I assumed this entry was going to be about pants as a synonym for crap, as in “that movie was complete pants,” which would have been much more interesting. I’ve never heard an American refer to underpants as “pants.”
These days more commonly rendered as “pance” in online blogs.
I always wonder why the north of England uses pants rather than trousers?
My mother was English and I have heard many of the “Britishisms” during her 100-years on earth. If both kay and Jan were a little older, they would be familiar with the term knickers that men wore up to the mid-forties in the Unites States. No, we are not cross-dressers, we merely wore the hand-me-downs from our older brothers who wore fashionable knickers.
RAM
In Australia we use “pants” to denote trousers, “underpants” to denote male underwear, and “panties” to denote female underwear. Simple. No ambiguity whatsoever!
Same as in U.S.!
In Doctor Who: Season 7, Episode 4, “The Power of Three,” Rory walks into the kitchen in his skivvies and announces, “There are soldiers all over my house, and I’m in my pants.”
In Britain men wear pants or underpants (but never knickers, cross-dressing aside) and women wear pants or knickers (but never underpants). Panties is sometimes used but generally only when describing women’s lingerie or ‘sexy’ underwear. Clothing shops always refer to women’s pants or knickers as ‘briefs’ but this hasn’t caught on outside of the retail environment. The word underpants is also more nuanced: the term tends to refer to what I believe AmE might call Jockeys or tighty-whities, close fitting stretchy briefs often with a hidden gap (sometimes referred to in Britain as ‘y-fronts’, although this seems to be used less nowadays as such items have become less fashionable). Boxer shorts, or boxers, is used in the same way as in AmE but boxers are not generally referred to as underpants. I have never heard ‘pants’ used to denote trousers in BrE.
I too live in Australia Philp Argy and I haven’t heard the word “panties” since I was very young (longer ago than I care to admit), and even then it was used by elderly relatives.
Men’s underpants are often referred to as “jocks” – a throwback to the original Jockey brand Y-fronts.
I wear knickers, my husband wears underpants, but I notice I sometimes refer to his as knickers as well.
Having lived in Yorkshire for 30 years, I’ve hardly ever heard “pants” used for “trousers”, but I think most Brits have been aware of US usage for several decades now.
It seems to me that for a while, aware of the potential confusion, trousers and underpants were used, at least for male clothing, ladies might wear jeans or slacks, with knickers or panties underneath, but to say “she wears the trousers in that house” is still somewhat derogatory for both members of a couple.
Out of interest, I’ve just looked at the mens’ underwear section of the Marks and Spencer website – probably the most archetypally British mainstream clothing retailer. They call things boxers, briefs, trunks, hipsters, slips, but no sign of either pants or underpants.
When I was young, in Scotland, my parents said underpants for the clothing I put on before trousers. This suggests that at some point the underwear I put on was clearly something to wear under my pants. So my take on this is migration to the Americas from Europe took words with casual meanings and continued the ambiguity. Clearly, the use of “pants” for trousers, in Scotland had ceased and in time the colloquial dropped “under” from pants as my Scotish wife will now say “here are your pants” and I know she means my underpants. Equally living in Devon as we now do, I know which bit of clothing my daughters describe when they say panties. They all know that pants means trousers for men in the American sense, but women all wear slacks, jeans, ski pants, snow pants, salopettes( snow pants) ……… I’m confused! So it’s clear that we English speakers change words frequently and there is no right or wrong. There is continuous evolution kept up to date courtesy of sex in the city, friends, and made in Chelsea, home and away, neighbors… Or Neighbours ….. ?? Etc!
Growing up in the 60s and 70s in NW England, I used trousers and pants interchangeably. The garments that you wear under your trousers were always underpants or undies.
Interestingly, although I think it is rare in the UK to refer to women’s undergarments as underpants, it is reasonably common to refer to women’s undies.