The meaning of MIA is “missing in action”.
The abbreviation comes from and is most prevalently used in military contexts, however, it is often encountered among gamers as well.
“Mia” was first used by the US military during World War 2, in 1944, to describe soldiers who have not returned from a mission and whose fate is not known.
Since then it became a well-recognized abbreviation in army jargon used in military operations to this day.
The spread of the internet and computing technologies in the end of the 20th century has led to the further spread of abbreviations.
In the last few decades, “mia” has been known to be used among gamers to describe someone who has other business to attend to and who is not available in-game.
It can be encountered in gaming forums, games themselves not to mention the military, where the phrase was created.