1. v. The act of exploiting a terminal which someone else has absentmindedly left logged on, to use that person's account, especially to post articles intended to make an ass of the victim you're impersonating. It has been alleged that the term originated as a reversal of the name of the gentleman who most usually left himself vulnerable to it, who also happened to be the head of the department that handled PLATO at the University of Delaware. Compare baggy pantsing.
2. n. The victim of an act of derfing, sense 1. The most typical posting from a derfed account read “I am a derf.”.