The Collaborative International Dictionary of English v.0.48:

Derf \Derf\, a. [Icel. djafr.]
   Strong; powerful; fierce. [Obs.] -- Derf"ly, adv. [Obs.]
   [1913 Webster]

The Jargon File (version 4.4.7, 29 Dec 2003):

derf
 /derf/

    [PLATO]

    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.?.