Note, on your keyboard, the location of two different punctuation marks—/, or forward slash, and \, or backward slash (also known as backslash). As you read from left to right, the forward slash falls forward while the backslash falls backward;

引用自 《learn c programming》

因为我们是从左向右阅读代码，/ 这个符号是向前倒下去，在中国的文化下就称其为正，所以叫正斜杠，\ 这个符号是向后倒的，在中国的文化下就称其为反，所以叫反斜杠。