Let's start with the more competitive AmE-to-BrE category. Here we've had some nice suggestions, and the hono(u)r nearly went to primary in the sense of 'preliminary election'. Reader-in-Ireland mollymooly had suggested this at the end of 2009, noting that the Conservative party had an open primary to choose a candidate for the House of Commons seat for Totnes. Perhaps it should have beaten staycation last year--but it came to my attention a little too late. But it was ousted as frontrunner in the last day of nominations, when SP nominated a gerund that has both been discussed in the news this year and made its way into UK news. And that gerund, The 2010 American-to-British Word of the Year is:
The word came into the news, of course, when President Obama said that the Democrats had taken a shellacking in the midterm elections. It made enough of an impression in its native US that it came 7th in Merriam-Webster's top 10 words of 2010. But it required even more looking-up in the UK. The OED lists it as 'originally and chiefly U.S.', and it also lists the plain verb, shellac (note the lack of k!), as originally and chiefly AmE (while the noun, for the varnish-type substance, is not dialectally marked). The BBC Magazine ran an article on 'What is a Shellacking?', David Crystal discussed it on Radio 4, Michael Quinion covered it on World Wide Words, and Jenny McCartney in the Telegraph thanked Obama for 'an extremely useful addition to the lexicon'--just to name a few UK commentators on the subject. One does seem to find shellacking in the UK sports press (especially regarding [BrE] football/[AmE] soccer) before Obama brought the word to public attention, but since Obama's statement, it seems as if the frequency of that usage has increased. For example, in the Guardian, there are seven uses in November and December, but only two in Sept/Oct. (However, there are five during the World Cup in South Africa and other clumps of them during the year.) A search for the word in UK political contexts shows up in colloquial contexts such as:
Like, for instance, his [Cameron's] current 'shellacking' (love that word) over a supposed lack of vision and confidence in the recent Guildhall speech. [Skol303 comment on Nick Robinson's blog]
Vince Cable being torn a new one by Kirsty Wark on Newsnight...she got him so rattled he developed a Herbert Lom-like twitch (left eye) halfway though the shellacking by Wark (I kid you not). [samandmai comment on digital spy]
So thank you, SP, for a fantastic nomination!
And on to the BrE-to-AmE winner. This is always a tougher category--in part, because I live in the UK, but mostly because of the lesser impact that UK news and popular culture makes in the US. The winner is not a particularly 2010 word--instead, it's one that's been making steady progress in AmE over the past decade. But in hono(u)r of the near-culmination of the Harry Potter film adaptations, the British-to-American Word of the Year is:
...in particular, the adjectival use to describe hair colo(u)r and, to some extent, the noun use to mean 'a red-haired person'. Twice this year I've heard from US parents (including Mark Allen) who have said that their children use ginger in this more British way because of the influence of the Harry Potter stories, which features the red-headed Weasley family, including Harry's sidekick Ron. (Here's my old post on the topic.) The much-discussed new Google n-gram tool shows 'ginger hair' steadily increasing in American English books since 1995, though Harry Potter was not released in the States till September 1998. In British English books, however, there's an increase in the Harry Potter days (after some years of decline), but what looks to be a decrease as we come toward(s) the present. It's hard to say if that's meaningful--and unfortunately I don't have access to any British corpus that takes us up to date. In the more reliable Corpus of Historical American English, there are 8 uses between 1940 and 1979, none in the 1980s, five in the 1990s and 8 in the 2000s, which seems to show the Harry Potter effect. It's harder for me to find incursions of the noun ginger in the meaning 'red-head' in AmE, since one must search for word strings, not meanings. All I can think to do is to note that the Urban Dictionary entry for the noun ginger include some contributions that spell color without a u. Further evidence is welcome in the comments.
Also welcome in the comments are your thoughts on whether I've done an effective or abominable job in choosing this year's Words of the Year. But if you don't like them and didn't nominate any, I reserve the right to roll my eyes at you. Through the computer. Ouch.