I first came across the term back in the days when Word Processing was the "IN" application. You even had dedicated computers that only did WP, such as those from Wang. Most computer screens were text only screens (usually 24 lines each of 80 characters) and only supported one style and size of font, but printers were capable of more complex output. So when you composed a word processing document, you embedded (though functions of the WP editor) codes in the text that told the printer that the next piece of text was to be underlined, or in a certain font, or a part of the document was to contain a drawing or image. So the document as it appeared on the screen was completely different to what was printed and you really could only see what the output was going to really look like, by actually printing it. Great for wasting paper.
Then as computer screens became more advanced and were capable of supporting graphics and any text style or font, the document as it appeared on the screen was able to resemble the document as it was going to be printed. So Word Processing programs that were able to display exactly like they printed were called WYSIWYG editors. What you see is what you get. All advanced WP programs today, like Microsoft Word or Apple iPages have WYSIWYG editors.