fiddly

adjective

fid·​dly ˈfi-dᵊl-ē 

chiefly British

: requiring close attention to detail : fussy

especially : requiring an annoying amount of close attention

… the tiny control buttons on the back are fiddly.—M. J. McNamara  

As a -y adjective, it's a little odd, since -y is usually added to nouns. Fiddly derives (according to the OED) from the verb fiddle 'To make aimless or frivolous movements' (OED), not the noun. That verb does ultimately come from the noun fiddle, but that's not what's relevant to a suffix that usually attaches to nouns. The suffix wants something that's a noun now, as in cinnamon-y or snowy.  But the OED tells us of -y adjectives:

The first OED citation for landslide (which it marks as "Originally U.S.") is from 1822; early citations are hyphenated, but the hyphen was soon lost. (That OED entry was updated in 2021.) The BrE word landslip, by comparison, dates back to at least the 1670s. While the OED marks landslip as 'also figurative' none of its examples are figurative uses (but that entry has not been updated since 1901; it is irregularly hyphenated into the 19th century). Here are the definitions:

landslide   1. The sliding down or subsidence of a large mass of earth, rock, etc.; a landslip; esp. a collapse of earth or rock from a mountain or cliff. Also figurative.  2. An overwhelming majority of votes for one party or candidate in an election; a victory achieved with such a majority.

landslip The sliding down of a mass of land on a mountain or cliff side; land which has so fallen. Also figurative and attributive.

In the last one there, the headline says landslip, but landslide is in the first line of the article.

To me, landslide sounds much bigger than landslip, and that might be reflected in the large in the former definition. Around here in the South of England, landslips occasionally close down rail travel between Brighton and London. In that case, it'll be that some earth has washed down from the slopes along the (BrE) railway line/(AmE) train tracks. There, I don't tend to hear landslide, and sure enough, those headlines tend to be about landslips.

By 2023, far more of the American fortnights seem to be homegrown. Many of those are about sport(s) and many of those are about European football (AmE: soccer). But a good few (like the nfl.com one above) are about US sports. It's possible that the sports pages, "a NOOBs hotbed" are the entry point for the current fortnight trend. 

One could think that the sports connection is what made Swift aware of the term—but I think it's a word that poetically minded and well-travel(l)ed Americans would often know. So I'm not going to bet that the inspiration for the word use was Swift's involvement with an NFL player. 

The song ends with some American geographical detail:

Thought of callin' ya, but you won't pick up
'Nother fortnight lost in America
Move to Florida, buy the car you want
But it won't start up till you touch, touch, touch me

It feels like the juxtaposition of fortnight and America is a nod to the unAmericanness of fortnight. 

Linguistic Americanness/Britishness depends on how you define Americanism and Britishism. This one is British because it died out in the US, not because it was never used there. Its new American fame is a tiny drop compared to its early-US use:

Note that fortnight been going down-down-down in the UK too. British people are saying two weeks more than fortnight since around the 1970s:

Some people call that Americani{s/z}ation. I'm not so sure. It's not like two weeks is a phrase an English speaker would have to learn from Americans. It wasn't Americani{s/z}ation when English speakers stopped saying sennight (='seven nights', like fortnight = 'fourteen nights') in the 17th century in favo(u)r of one week or a week. It's just using another, more transparent expression that your language allows, and allowing the more old-fashioned-feeling one to fall away. 

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At this point, I am not certain there will be a US-to-UK Word of the Year 2024. If you're reading this before I post one, you're still welcome to nominate! 

[I]n American English, right conveys the speaker's knowing stance and, in certain environments, the speaker's claim of primary knowledge. In contrast, in British English, right registers provided information as previously unknown, informative, and relevant to the current speaker's ongoing project. 

        [...] 

[S]ome UK usages of right—such as registering of potentially consequential information and projecting a transition—are quite similar to US okay in comparable positions [...]. This suggests a possibility that, in US English, okay took over some of the right usages and/or, in UK English, right took over some of the okay usages."

Their research was inspired by this interaction between BrE-speaking "AB" and AmE speaker "GA":

So, essentially, the British use of right in that context leads GA to think that AB is confirming (rather than acknowledging receipt of) the information. If AB had said OK, then GA would have understood it as acknowledgement rather than confirmation.

Even though the researchers note differences in usage between BrE and AmE okay (though keep in mind that their research is about right), it seems like a fitting US-to-UK WotY because (in whichever usages), it's used more than ever in the UK. Here it is in the British section of the News on the Web corpus, where it shows OK and okay climbing in the last couple of years.