In the U.S., military terms were common in everyday life during the 1950s to 1980s, because most older men had served in the military during World War II (1940s) and some younger men had served in the Korean War (1950s) or the Vietnam war (1960s-1970s). Today, military terms are relatively rare in civilian life.

I always thought it was “Away Without Official Leave”. But I was never in the military so I don’t know for sure. It can be used in non-military contexts in a joking manner. For example, if there is a friend you used to always hang out with at a certain bar, but they haven’t been coming around for a while. Then one day they show up again, you can say to them, “Hey, man! Where’ve you been? You’ve been AWOL!” and it would mean something like “Great to see you! I have missed hanging out with you! Remember when you used to come here more often?” It’s not used in an “official” sense outside of the military. But most people will know what it means, so it can be used jovially.