Meaning

noun a photocopy made on a Photostat machine

noun a duplicating machine that makes quick positive or negative copies directly on the surface of prepared paper

verb make a copy by means of a Photostat device

originA genericization from the trade name of one of the earliest commercially successful types of photocopier, a pre-xerography type. Xerography displaced it in the 1960s and 1970s; since then the relics of it are historical curiosities. More in Wikipedia at Photostat machine.

Etymology from Wiktionary, CC BY-SA 3.0