Separated by a Common Language

• I'm only talking about the relationship between British and American English here (as is my theme). These expressions may well have equivalents in other languages or dialects.  
• By Untranslatable I mean that there is no lexicali{z/s}ed equivalent in the other dialect. And by lexicali{z/s}ed I mean that the expression is a word or an idiom--something that language users learn through hearing others say it, rather than something that has been made up anew.
  One can translate things by making up new sentences or phrases that describe the same thing, sure. But it's special when a language has lexicali{z/s}ed an  expression for something--it tells us something about the culture that invented and uses that expression.
• Many of these have started to be borrowed between the dialects--and that's natural. If it's a useful expression and the other dialect doesn't have it, it's a prime candidate for international migration.
• If you have not heard of the word before (even though I've said it comes from your country), this is not cause for complaint. It is cause for celebration that you have this opportunity to enrich your vocabulary! 

• They can't repeat items from the previous Untranslatables Months.
• It should be the expression that's missing from the other country, rather than the thing. So, for instance Page 3 Girl was suggested, but there is no American newspaper that puts topless young women on page three every day (thank goodness). There's no word for it in the US only because there's nothing for it to refer to in the US, so it doesn't belong in this particular list.
• I try to alternate American and British expressions (but that doesn't always work out).

• BrE snug: a small, comfy room in a pub. Occasionally  extended to other comfy personal (orig. ScotE) hidey-holes. Here's a Wikipedia description. [I learned this during the year while reading an article that I now can't find. I had to look the word up, and then spent the rest of the year waiting for untranslatables month to come round again.]
• AmE to jaywalk: to cross the street/road against the light or where there's no crossing. Thanks to @SimonKoppel for the suggestion. As I noted in a later tweet, this word is known by many in the UK, but generally only used to refer to people doing it in the US. Some British twitterers objected that this couldn't count because the thing doesn't exist in the UK. They were under the impression that one cannot jaywalk in the UK because it's not illegal to cross in the middle of the (orig. AmE) block here.  But notice that there's nothing about legality in the definition I've given. I grew up in a place where (I was told, I've never actually checked) jaywalking wasn't illegal. But we still called it jaywalking. (Remember: laws--including many traffic laws--vary by state in the US.)
• BrE Billy No-Mates: a friendless person. Here's a history of the phrase. (Can't find who suggested it, but thanks!) Several people sent variations on this like Johnny No-mates, Norma No-Mates and Norman No-Mates, but Billy seems to be the original (and the one I hear most--the others may be a bit more spread around the anglophone world).
• AmE backwash: saliva/mouth contents that go back into a bottle that's been swigged from. (Urban Dictionary's take on it.) Several Brits told me they knew this from childhood, but it's still not (in my experience) widespread in the UK. Of course, the word-form is used in both dialects for other kinds of washing-back in rivers and plumbing.
• BrE garden(ing) leave: Explained in this old post.  Thanks again to @SimonKoppel.
• BrE to plump for: to choose suddenly after much dithering. Thanks for the suggestion to @rwmg.
• AmE will call: [of tickets] to be collected at the box office. Wikipedia says COBO ('care of box office') is the BrE equivalent, but it's not in general use. In a US theat{er/re} you might have to go to the will-call desk/counter/box office to get the tickets. COBO isn't used like that. Yet another one suggested by @SimonKoppel. I might have to put him in charge of Untranslatables month next October.
• BrE to decant: to transfer people temporarily to another location. See sense 1.1 in Oxford Dictionaries Online. Thanks to Diane Benjamin for this suggestion.
• AmE to stop on a dime: to come to a halt quickly and neatly in exactly the right spot. Many complained that this has a BrE equivalent in stop on a sixpence. Fair enough. Though I will note that turn on a sixpence seems to be more common than stop on...
• BrE three-line whip: Party instruction to Members of Parliament that they must vote with the party on some matter. (Here's more explanation from a Stack Exchange.) There is a question here whether it should count: is there an equivalent three-level structure of whips in the US? Well, there could be, so I gave it the benefit of the doubt. Thanks to @JanetNorCal for the suggestion.
• AmE loaded for bear: well prepared (and probably eager) for a forthcoming confrontation. Thanks to @sethadelman for the suggestion.
• BrE gazunder: [for a buyer] to reduce an agreed-upon price for a house/property just prior to signing contract.  Here's Word Spy on it.  
• BrE gazump. To obtain a property by offering more for it than an already-accepted offer. Here's Oxford Dictionaries Online on it.
• AmE layaway (= AusE lay-by). Instal(l)ment purchasing, where the item's not received until it's paid off. There was some discussion about whether this should count because it's unclear that the equivalent exists in the UK. British hire-purchase is the equivalent of AmE rent-to-own or rental-purchase, in which case you take the thing home and make payments on it. I allowed it because I think one could argue that certain Christmas schemes in the UK (like this one) are kind of like layaway. Thanks to @smylers2 for the suggestion.
• BrE U and non-U: (Non)-upper class, with particular reference to words that "should" or "shouldn't" be used. Here's the Wikipedia article on it. And here are places where the distinction has been mentioned on this blog.
• AmE charley horse. A cramp in the leg. Here is Merriam-Webster's definition. Thanks to @meringutan for the suggestion. There were some suggestions for British-dialectal equivalents of this. Hard to tell if they're really equivalent. You can discuss amongst yourselves in the comments.
• BrE WAGs: wives and/or girlfriends of (BrE) footballers as a type of celebrity. Discussed on this blog here. Thanks to @meringutan.
• AmE snow day: a day when schools and businesses are closed due to snow. (Longman definition). Sometimes heard in UK now, but no local lexical equivalent. Thanks for the suggestion, @laurelspeth.
• BrE chav. This is a word for a stereotyped type of person. Here's Wikipedia's take on it. Suggested by @kearsycormier (thanks!). This one I was most uneasy about including, because I think it is the case of it being more the referent (in this case people rather than things) rather than the word that the US lacks. It's all about the UK social class system, which operates in different ways, with different emblems, than the US class system.  Many years ago I wrote about an attempt to import chav to the US. It hasn't worked.
• AmE family-style: adjective or adverb describing the serving of food at restaurant in dishes that are to be passed (a)round and taken from, like at home. (Oxford's definition)
• BrE scrumping: stealing apples from an orchard. Thanks to @beardynoise for the suggestion.
• AmE palimony: (humorous) alimony-style payments made after the break-up of a non-marital relationship. 
• BrE dodgy: with its many shades of meaning, it's hard to think of an exact equivalent: Here's Oxford Dictionaries Online on it. Once one learns this word, it soon becomes a necessary part of one's vocabulary, so it's not surprising that there are US sightings of it. Thanks to  @tonythorne007 for the suggestion.

Hospitals across Sussex were inundated with patients over the weekend who had broken limbs after falling on ungritted pavements.  [Ed. note: the weekend broke limbs?]

Dozens of people contacted The Argus to condemn the lack of gritting which has left many elderly people trapped in their homes.  [Ed. note: did they also condemn the lack of a comma on a non-restrictive relative clause?]

A Brighton and Hove City Council spokesman said all the authority's refuse and recycling staff were being diverted to gritting roads and pavements today.

In the United States, the National Weather Service defines a blizzard as sustained winds or frequent gusts reaching or exceeding 35 mph (56 km/h) which lead to blowing snow and cause visibilities of ¼ mile (or 400 m) or less, lasting for at least 3 hours. Temperature is not taken into consideration when issuing a blizzard warning, but the nature of these storms is such that cold air is often present when the other criteria are met.^[1] Temperatures are generally below 0 °C (32 °F).

According to Environment Canada, a winter storm must have winds of 40 km/h (25 mph) or more, have snow or blowing snow, visibility less than 500 feet (150 m), a wind chill of less than −25 °C (−15 °F), and all of these conditions must last for 3 hours or more before the storm can be properly called a blizzard.

Many European countries, such as the UK, have a lower threshold: the Met Office defines a blizzard as "moderate or heavy snow" combined with a mean wind speed of 30 mph (48 km/h) and visibility below 650 feet (200 m).