AFK or Away From Keyboard is simply a popular shortening among gamers, However on the also popular IRC Chat. <Zanthis(ALE)> originally started the AFK Tornado craze when he originally set his status as this due to an actual Tornado in 2005. I believe the original quote is posted on Bash.org.
I hope this little background history was helpful, for those Newfags out there its a very "you had to be there moment" type situation.
<Zanthis(ALE)> AFK Tornado