It's funny to see a British slang expression making its way into American discourse. In a review of Dawkins's Greatest Show on Earth, Nicholas Wade writes:
There is one point on which I believe Dawkins gets tripped up by his zeal. To refute the creationists, who like to dismiss evolution as “just a theory,” he keeps insisting that evolution is an undeniable fact. A moment’s reflection reveals the problem: We don’t speak of Darwin’s fact of evolution. So is evolution a fact or a theory? On this question Dawkins, to use an English expression, gets his knickers in a twist.Wade's review has generated a huge number of responses, one of them from Duke professor Alex Rosenberg, which enjoyably plays with the "knickers in a twist" expression:
Since Nicholas Wade, in his review of “The Greatest Show on Earth: The Evidence for Evolution,” by Richard Dawkins (Oct. 11), has accorded to us philosophers of science the role of arbiters in deciding on the “cognitive structure of science,” let me suggest that it’s Wade who has gotten his knickers in a twist and not Richard Dawkins.I smiled to see the very American English "gotten" next to "his knickers in a twist", and I quite like it. The last line, on the other hand, tries to play with the image a little and it comes out a little strangely, at least to these British ears. The expression "getting your knickers in a twist" means something like overreacting to something relatively minor, "getting in a tizz" about something. The actual "twisting" itself is not material, as it were.
Evolution is a fact, natural selection is a process and Darwin’s theory is that the fact is explained by the process. The facts of evolution are as evident as any facts about the past can be . . .
. . . . Wade writes: “Creationists insist evolution is only a theory, Dawkins that it’s only a fact. Neither claim is correct.” Like too many journalists lately, even in The New York Times, Wade seems to think that the appearance of balance requires that he condemn with fine impartiality creationists and Dawkins. The twists into which Wade has to contort his review in order to do this will make for needless knicker-untwisting.
A little googling shows that there is a comparable expression over here in the US, "get your panties in a knot". It's not something I have heard, though, and it sounds like an attempt to carry the expression over, "panties" being AmE for BrE "pants" or, as here, "knickers". "Panties in a knot" is clearly not as memorable as "knickers in a twist".