AFK
Away From Keyboard — "back in a bit", one of chat culture's oldest abbreviations.
Born on BBSes and IRC in the 1980s and 90s, where announcing that you were stepping away was simple courtesy rather than just going quiet. The neighbouring BRB (be right back) means roughly the same thing but promises a return.
It carried over into gaming, where an AFK player is a problem rather than a politeness — most games now kick idle players automatically. In recent years it has taken on an ironic use for being offline in general, the "touch grass" variety.