Definitions for jack in
To stop doing a regular activity. Often a job or studies.
To insert an electronic coupling into a receptacle; to connect to something, whether involving a physical medium or not.
The New Hacker's Dictionary
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.