The very early copiers used the camera technique for making copies of the document. The camera will take a "photo" of the document and the photo was developed using a toner. Therefore the term "photocopy".
Xerox Company of USA was the poineer in developing the electronic copy of the document which was much higher in quality than a photocoy. Xerox then used the slogan " Dont just photcopy, Xerox it". Xerox meant an electronic copy without involving a photo.
The photocoying and Xeroxing then became synonymous. But at the same time, terms like poor xeroxing ,poor xerox copy also came into being. May be Xerox copmany did not like the word "poor" before it and sued the company.