Source : Free On-Line Dictionary of Computing

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}]