What's a "Xerox word"?
Solution 1:
Where a brand name becomes known so well, it becomes used in place of the product. Kleenex is often used instead of tissue. In this case, Xerox is a brand of photocopier but Xerox gets used instead of the word copy. So the stammering of I, I.. is a form of 'speech' copying. The author was coining the stammering as making Xerox copies.