On the TV show Slow Horses (set in the UK), a character announced he was going to get some kip, and the meaning completely eluded me.
Likewise when, ages ago in a bar, some friends mentioned totty.
We are truly separated by a common language.
What do you think are the most impenetrable Americanisms/Britishisms for those on the other side of the pond?
Calling the main meal an entree is the one I find most bizarre.
I don’t know if it’s impenetrable exactly, but if you’re not aware of it and/or not being careful, the divergent definition of the word “fanny” can get you into serious trouble.
Also, in the UK, what an American calls “pants” are worn under the pants, which are called trousers. In the US, by contrast, you can say trousers, but you’re more likely to say pants, and under your pants, you (obviously) wear underpants. And back in the UK, a Brit will hear this usage, and think American vocabulary is pants.
Don’t ask Brits where the bathroom is unless you actually intend to take a bath.
And what they call bacon doesn’t look like bacon I am used to. More of a combination of a piece of bacon connected to a piece of ham.
Is that an Amerianism or a Britishism? That’s not quite the way I’ve heard “entree” used in the U.S.
An Americanism. No Brit would call the main course an entree.
‘Sketchy’ and ‘Mooch’ have tripped up a few Americans when I have used them; the most common meanings appear to be:
UK:
Sketchy: Vague, poorly-defined, inexact (figuratively: as though roughly sketched, rather than clearly drawn).
Mooch: To wander or amble without urgency or clearly defined purpose.
USA:
Sketchy: Unsafe, dishonest, untrustworthy.
Mooch: To beg, or borrow without any intent to repay; to exploit the generosity of others.
I think both sets of meanings exist on both sides, but there is a difference in the default/common meaning.
There’s also waffle:
Brit: drone on about something interminable
American: Fail to make a decision and stick to it.
Oh, I suppose a mandatory inclusion in this thread is pudding. It can mean a wide range of things in the UK. Here is an incomplete list:
Pudding (as a word on its own): The dessert course of any meal
Pease pudding: Dried yellow split peas cooked to a savoury paste (often cooked alongside a boiled ham so as to acquire some of its flavour)
Yorkshire pudding: Batter baked until light and puffy
Christmas pudding: Basically, a sort of moist, hot, rich steamed fruitcake
Bread & butter pudding: Sweetened French toast with raisins, baked in a dish
Bread pudding: Similar to the above, but with the bread broken up into pieces, spiced, and the the whole thing baked into a loaf
Rice Pudding: Short grain rice cooked (usually baked in a dish) with milk, cream, butter, sugar and spices until thick and creamy
Summer pudding: Cooked fruits (usually berries) encased in bread or cake, saturated with fruit juice, sometimes set with gelatin
Bakewell pudding: A tart made from puff pastry, filled with jam and almond-infused custard (exact ingredients subject to debate)
Sticky toffee pudding: Hot cake, with a warm caramel sauce
Eve’s pudding: Sponge cake baked in a dish with a layer of apple underneath
Malvern pudding: Apples baked in a sweet, spiced white sauce
Queen of Puddings: Custard, fruit and cake or bread crumbs, baked with a meringue topping
Suet pudding: A dome-shaped sort of cake/pastry, cooked by steaming; may be served with syrup, jam or other sweet stuff
Steak & kidney pudding: As above, but not sweet, and filled with meat and gravy
Black pudding: Blood sausage (the original ‘pudding’, since the word derives from the French ‘Boudin’)
White pudding: Sausage, similar to the above, but with less blood
Hog’s pudding: Like white pudding, without any blood (basically a particular kind of sausage)
Notably absent from UK ‘pudding’ terminology is the milk custard confection known in the USA as ‘pudding’ (where indeed, it may be the only meaning of ‘pudding’).
Also flan - in the UK, it’s pretty much a synonym for tart - a shallow, usually open-topped pie, with a crust, with either a sweet or savoury filling.
I understand that the USA meaning of flan is what we call creme caramel.
Also custard - In the UK, this is typically thickened with cornstarch and may not contain egg (this is a second layer of confusion that occurs when I use the term ‘custard’ to describe USA pudding).
We do have egg custard too, but custard can be either.
Colours associated with political parties in the USA are weird to me. Left is red here and right is blue.
Have the distinctly different meanings of pissed and knock up on either side of the Atlantic become common knowledge by now?
Fag -
Brit:
American:
Ever notice how the Brits have the most whimsical names for car parts? It’s like they’re in on a secret mission to make every vehicle sound like it’s about to attend afternoon tea. Take the hood, for instance—they call it a “bonnet.” I mean, who wouldn’t want their engine sporting a stylish little hat? And the trunk? Oh, that’s a “boot,” because apparently, British cars are always stepping out in fashion-forward footwear. As for “wings” instead of fenders? Well, clearly, British cars are just waiting for their moment to soar!
And let’s not even talk about how they spell “tyre.” They just threw in a ‘y’ to make it look extra posh, like they sat down one day and thought, “The word ‘tire’ feels a bit… tired. Let’s jazz it up with a bit of flair!”
My mum, bless her, was a British war bride from WWII, and we had to teach her proper English. 
What is a hood?
“I’m from the hood” (neighborhood). That’s urban cool.
Hood noun
So, a stylish little hat.
If anyone’s interested, there’s a youtube channel called Lost In The Pond that covers exactly this topic. It’s done by a guy that’s spent half is life in Britain and half of it in the midwest USA.
“The rapper wore a hood (hoodie) during his performance” sounds better than “the rapper wore a bonnet during his performance.”
This is the Dope, we fight ignorance proudly and openly.
So how has nobody yet had the courage to say “cunt” ?
Brits
use it as a mild, slightly impolite word.
I think they use it to describe any person, male or female, who is a bit irritating to talk to, maybe a bit obnoxious, or, at worst, acting stuck-up and condescending to you.
Americans…don’t use it that way. 
Yeah, and if you’re gonna carry a pack there, I do prefer the Britishim “bum bag”.