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

Get a life!
 imp.

    Hacker-standard way of suggesting that the person to whom it is directed
    has succumbed to terminal geekdom (see geek). Often heard on Usenet,
    esp. as a way of suggesting that the target is taking some obscure issue of
    theology too seriously. This exhortation was popularized by William
    Shatner on a 1987 Saturday Night Live episode in a speech that ended ?Get a
    life!?, but it can be traced back at least to ?Valley Girl? slang in 1983.
    It was certainly in wide use among hackers for years before achieving
    mainstream currency via the sitcom Get A Life in 1990.


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

Get a life!

    Standard way of suggesting that someone has succumbed
   to terminal geekdom.  Often heard on Usenet, especially as
   a way of suggesting that the target is taking some obscure
   issue of theology too seriously.  This exhortation was
   popularised by William Shatner on a "Saturday Night Live"
   episode in a speech that ended "Get a *life*!", but some
   respondents believe it to have been in use before then.  It
   was certainly in wide use among hackers for at least five
   years before achieving mainstream currency in early 1992.

   [Jargon File]

   (1995-01-18)