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

jack in
 v.

    To log on to a machine or connect to a network or BBS, esp. for purposes
    of entering a virtual reality simulation such as a MUD or IRC
    (leaving is ?jacking out?). This term derives from cyberpunk SF, in which
    it was used for the act of plugging an electrode set into neural sockets in
    order to interface the brain directly to a virtual reality. It is primarily
    used by MUD and IRC fans and younger hackers on BBS systems.


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

jack in

   To log on to a machine or connect to a network or BBS,
   especially for purposes of entering a virtual reality
   simulation such as a MUD or IRC (leaving is "jacking
   out").  This term derives from cyberpunk SF, in which it was
   used for the act of plugging an electrode set into neural
   sockets in order to interface the brain directly to a virtual
   reality.  It is primarily used by MUD and IRC fans and younger
   hackers on BBS systems.

   [Jargon File]