MIA means "missing in action" and can be either used literally or figuratively. Though most of us know the term in a lighthearted context, the term "MIA" actually gets its roots from military classifications. The term refers to members of the military who are reported missing during war or a ceasefire. They may have been killed, wounded, captured, or deserted, but their status is unknown. In modern-day slang, the term is used figuratively for unexpected absences to say you haven't seen someone in a while or can't find them.

Source: USA Today