V.E.R.A. -- Virtual Entity of Relevant Acronyms (February 2016):

AFK
       Away from Keyboard (slang, Usenet, IRC)


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

AFK


    [MUD] Abbrev. for ?Away From Keyboard?. Used to notify others that you will
    be momentarily unavailable online. eg. ?Let's not go kill that frost giant
    yet, I need to go AFK to make a phone call?. Often MUDs will have a command
    to politely inform others of your absence when they try to talk with you.
    The term is not restricted to MUDs, however, and has become common in many
    chat situations, from IRC to Unix talk.


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

AFK

    away from keyboard.