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

AFJ
 //, n.

    Written-only abbreviation for ?April Fool's Joke?. Elaborate April Fool's
    hoaxes are a long-established tradition on Usenet and Internet; see 
    kremvax for an example. In fact, April Fool's Day is the only seasonal
    holiday consistently marked by customary observances on Internet and other
    hacker networks.


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

April Fool's Joke
AFJ

    (AFJ) Elaborate April Fool's hoaxes are a
   long-established tradition on Usenet and Internet; see
   kremvax for an example.  In fact, April Fool's Day is the
   *only* seasonal holiday marked by customary observances on the
   hacker networks.

   (1995-01-25)