From LQWiki

AFK is a MUD abbreviation 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.

This article is based, in whole or in part, on entry or entries in the Jargon File.