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

J. Random Hacker
 /J rand'm hak?r/, n.

    [very common] A mythical figure like the Unknown Soldier; the archetypal
    hacker nerd. This term is one of the oldest in the jargon, apparently going
    back to MIT in the 1960s. See random, Suzie COBOL. This may originally
    have been inspired by ?J. Fred Muggs?, a show-biz chimpanzee whose name was
    a household word back in the early days of TMRC, and was probably
    influenced by ?J. Presper Eckert? (one of the co-inventors of the
    electronic computer). See also Fred Foobar.


The Free On-line Dictionary of Computing (30 December 2018):

J. Random Hacker

    /J rand'm hak'r/ MIT jargon for a mythical figure;
   the archetypal hacker nerd.

   This may originally have been inspired by "J. Fred Muggs", a
   show-biz chimpanzee whose name was a household word back in
   the early days of TMRC, and was probably influenced by
   J. Presper Eckert (one of the co-inventors of the electronic
   computer).

   See random, Suzie COBOL.

   (1996-10-16)