afk

from Jargon File (4.4.4, 14 Aug 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.

from The Free On-line Dictionary of Computing (8 July 2008)

AFK

   <chat> away from keyboard.

from V.E.R.A. -- Virtual Entity of Relevant Acronyms (June 2006)

AFK
       Away from Keyboard (slang, Usenet, IRC)