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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.
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