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

J. Random
 /J rand'm/, n.

    [common; generalized from J. Random Hacker] Arbitrary; ordinary; any one;
    any old. ?J. Random? is often prefixed to a noun to make a name out of it.
    It means roughly some particular or any specific one. ?Would you let J.
    Random Loser marry your daughter?? The most common uses are ?J. Random
    Hacker?, ?J. Random Loser?, and ?J. Random Nerd? (?Should J. Random Loser
    be allowed to kill other peoples' processes??), but it can be used simply
    as an elaborate version of random in any sense.


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

J. Random

    /J rand'm/ (Generalised from J. Random Hacker)
   Arbitrary; ordinary; any one; any old.  "J. Random" is often
   prefixed to a noun to make a name out of it.  It means roughly
   "some particular" or "any specific one".  "Would you let
   J. Random Loser marry your daughter?"  The most common uses
   are "J. Random Hacker", "J. Random Loser", and "J. Random
   Nerd" ("Should J. Random Loser be allowed to gun down other
   people?"), but it can be used simply as an elaborate version
   of random in any sense.

   [Jargon File]