Separated by a Common Language

On Twitter, I usually post a 'Difference of the Day' between British and American English every weekday. But for the past five Octobers, I've done the Untranslatable of the Day.
The moment I start tweeting about 'untranslatables' I expect to receive tweets and emails complaining about the concept, particularly that 'nothing is untranslatable'. That's why I write this self-plagiarizing introductory blog post each year. 

Yes, 'untranslatable' is not a very useful concept. I use it because it's shorter and more familiar than what I really mean: 'Lexicalized in a particular variety of English, but not another' That is, the concept may be expressable in the other English, but it hasn't been packaged as a lexical item—i.e. a word or an idiom—in every variety of English.

• 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), then I hope that you might celebrate that you've learned a new expression, rather than complain to me that it's not 'really' American or British. Please know that I'm not posting them without some research, and none of us has a complete vocabulary. That said, if you can improve on my definitions, challenge the 'untranslatability' or give other insight into the untranslatables, please let me know!

• If it's a word for a thing that isn't found in the other country, it doesn't count. That is, it's not Americans don't have a word for Eccles cake, it's that Americans don't have Eccles cakes. 

• I'm grateful for suggestions of additional untranslatables (though they may not make UotD status until next year), but I won't repeat any expressions that have been used in previous Octobers. The lists for each October are accessible by clicking on the 'untranslatable' label in the right margin, the bottom of this post, or, conveniently, here: untranslatable.
  There are also search boxes at the top and in the right margin of this blog. (The one in the margin works much better.) So please have a quick search before making suggestions, in order to cut down on the time that I spend responding to suggestions. (This is all voluntary on my part, please remember!)

Each year I've wondered: can I really keep this up for a(nother) month? Are there that many concepts that are put into words or idioms in Britain or the U.S., but not the other country? Well, we've come up with more than 100 so far, and this year, I kept a file of UotD suggestions all through the year and can say with confidence that there are enough for a sixth go-round. But unlike in other years, I've not been able to balance the number of British and American untranslatables. I've got lots of British ones. Please feel free to send more American ones my way!