AFK

Away From Keyboard. Commonly used abbreviation in online games to tell other players that one is away for a while.

Emote
If you are going to be away from Guild Wars for a while (toilet, call, phone call etc) you can use the /afk Emote in the chat window to place your character in a sitting position and let other players know that you are AFK, the /afk emote looks the same as /sit.