SIA is basically a Gaming term in computer world, refering to real life term - KIA.
It's very alike with word skillad
, meaning Skilled In Action. It's Killing with Skill = S-killing.
Being SIA in first person shooters has a meaning of being shot in a very unorthodox, very thought through and amazing way.
SIA can be anything including Headshot
s, and other amazing stuff that can be done using skill.
SIA'd also may have a meaning of " Served in action " linking to " you got served " .