As mentioned 27 times above afk refers to "away from keyboard". But seeing I first encountered this term in the german version of an online game designed by a company interested in weather turbulences, I took it to be a shortening of "auf klo" meaning "in sitting position, currently getting rid of the pizza I had for lunch". In 49% of all cases, this is an accurate rendering of the term when used in online communication situations. This word is not only used in written language but also in spoken. Anyway, in German the term has undegone significant extensions, resulting in:

with suffix
- afklo, also afkaken (as explained above)
- afkleenex (getting a tissue, winter phenomenon)
- afkaffee ( 49%, equal score to afklo, possible connection still to be researched.)

with prefix
- brain-afk ( used in in-game situations, work out meaning for yourselves. Being in a different category, this meaning is not included in the statistics above but would score significantly higher than either of the other two major forms.)

Away From Keyboard.

Away From Keyboard : often used in online gaming when someone is not moving they are assumed to be afk.

Abbreviation for "Away From Keyboard" This, as well as other abbreviations were developed to speed up communication using 300 baud modems in the early to late 80's.

Away From Keyboard. Almost synonymous with Masturbating.

It means: "Away from Keyboard", usually used in games.

a term heavily used in online video games to mention the player will be away for a certain period of time. AFK can be both "Away From Keyboard" or "A Free Kill".

"Away From Keyboard"
This means that you are temporarily away from the keyboard during an online game with other humans.