美国人在打棒球时经常喜欢发曲线球，使球的方向会发生突变，这样给对方造成错觉，不能回击那个球。这和其他球类运动，如排球、乒乓球或网球，在发球时发转球的作用也很相似。逐渐地，这种让对方产生错觉、因而无法应付的球艺变成了日常用语：to throw a curve。

Curve就是“曲线”的意思。To throw a curve就是“在打棒球时发曲线球”。但是作为俗语，它的意思是“给别人造成错觉，让他难以应付”。下面是一个刚刚考完历史的学生在埋怨他的教授：

Professor Green certainly threw us a curve today. Half the questions were from parts of the book he told us to read but he never covered them in class. Of course, most of us didn't bother to read the stuff, so we really got caught.

格林教授今天可真是把我们难住了。考题的一半是他要我们看的那本书里的内容，可是他从来也没有在课堂上讲过。我们大多数人也都没有花时间去看那玩意儿，所以今天我们突然发现无法应付考题了。

To throw a curve还可以解释为“欺骗”，下面就是一个例子：

My supervisor¹ threw me a curve when he told me he would recommend me for the promotion². He didn't tell me he was also recommending three other people for the same job. So I didn't get it.

我的上司欺骗了我，他告诉我他要建议给我提升。可是，他没有告诉我他同时还推荐了另外三个人担任同一个职位。结果我没有被提升。