derf
noun: An inept or foolish person, someone who doesn't use their head. On the PLATO computer system of the 1970s (which in many ways foreshadowed the Internet), a derf was one who left their computer terminal without logging out. 

verb: On PLATO, a prank played on a derf. Sending out unpleasant or embarrassing messages under the handle (or "signon") of a person who'd forgotten to log out. Or ruining programs they've been working on.
noun: Wow, "Melvin of MIT" is a real derf! Look, he just walked out and left his terminal logged in.

verb: Hey Melvin, I just tried to run one of your programs and it looks like somebody got into it and put in a bunch of porn. Looks like you got derfed!