The Word Detectiveの受け売りですが、分かりやすくいえば、ここでいうloopとはローラー・コースターが宙返りする、あのループ状の部分で、あそこを通るときは天地が逆さまになります。そこから、うろたえる、びっくりするという意味が出てきたようです。
詳細は下記に引用したThe Word Detectiveの記事を参照して下さい。さらに興味があれば、下記のURLからサイトまで行ってみて下さい。
Dear Word Detective: I teach a class of "native" English speakers here outside Tel Aviv. The kids do just fine with ordinary readin' and writin', but explaining idioms is a real challenge. How about some help with "thrown for a loop." I can't find anything about it in my (limited) library. -- Janet Nusbaum, via the internet.
snip
I presume that you have already explained that "thrown for a loop" means "bewildered" or "dazzled" or, less frequently, "defeated." The question is why it means that.
The answer lies in what the Oxford English Dictionary calls, for some mysterious reason, a "centrifugal railway," but which we Americans know by the much livelier name "roller coaster." The "loop" in a roller coaster or other carnival ride is that portion of the track when the cars travel up in a circular, twisting motion so that at the apex the passengers are traveling upside down, a process also known as "looping the loop."
It is this image of "looping the loop" that underlies "thrown for a loop," the metaphor being that some news or event is sufficiently shocking as to throw the person upside down into the air in a looping motion. This all sounds terribly dry, but you've probably seen this event demonstrated literally in hundreds of cartoons. In fact, since "thrown for a loop" first appeared in about 1923, cartoons may actually have boosted the popularity of the idiom.
The Word Detective
URL: http://tinyurl.com/rbkex
お礼
完璧な回答,ありがとうございました。このサイトも役に立ちそうなので活用していきたいと思います。