Lynne Murphy’s new book, The Prodigal Tongue, has plenty of blog-fodder, which I’m just starting to make my way through. As with “bestie,” I was surprised when she mentioned “flummox” as a Britishism, but once again, she’s right. For the most part.
It’s a word with a history, for sure. The OED categorizes is as “colloquial or vulgar” and gives as primary definition: “To bring to confusion; … to confound, bewilder, nonplus.” The first citation is a line of dialogue from Dickens’s Pickwick Papers, 1837: “He’ll be what the Italians call reg’larly flummoxed.” (There is no evidence that the word has any Italian derivation–it shows up in late nineteenth-century books described as provincial English slang.) Green’s Dictionary of Slang bests that by three years with this quote from the bawdy songbook Delicious Chanter: “Joe owned he was flummix’d and diddles at last.”
However, both Green’s and the OED note a roughly contemporaneous use of the word in the United States, with the meaning “give in, collapse.” The OED has this quote from the 1839 novel Green Mountain Boys, by the Vermont author Daniel P. Thompson: “Well, if he should flummux at such a chance, I know of a chap..who’ll agree to take his place.” The online version of Green’s Dictionary of Slang has a definition and citations (the first from Britain, the second from the U.S.) for another meaning of the word.
I also found this “flummox” in a slightly later novel by Thompson, Locke Amsden, or The Schoolmaster: “‘Well, he was a mean scamp, for all that,’ replied the oldest boy; ‘and we should have shipped him, at one time, if some of the boys had not flummuxed from the agreement.”
In any case, the “confound” meaning and the “flummox” (rather than “flummux”) spelling got solidified in Britain and seem to have been taken up in the U.S. in the middle of the twentieth century. An early use in the New York Times came in a 1954 James Reston column: “The Democrats were frankly flummoxed tonight.”
As this Google Ngrams Viewer chart suggests, U.S. use began to really rise in the 1960s and caught up with and then surpassed British use just before the turn of the twenty-first century:
notoneoffbritishisms.com 
I have just started reading the excellent Prodigal Tongue
Thank you, Ben, for making me aware of its existence.
As a Canadian who works in government and the judicial system I am regularly flummoxed – in fact received an “award” two years ago after a particularly tricky project and it was called “The Golden Flummox – but I only know the UK sense of it as being confounded or completely taken aback.
I wonder if the increase in US usage of “flummoxed” was due to a generation of Americans reading The Hobbit at a young age and thereby learning the word.