absent without leave
additionally a.w.o.l., military initialism (acronym) for absent without leave. The -o- is apparently indeed there mainly so the assemblage may be pronounced as a word. In U.S. military usage at least from World War II, preferred usage by 1960.
absent without authorization
- person who is away or absent without leave
brief for missing Without keep, AWOL defines any person whom left without warning or permission in talk or other text-based conversation. Under is a typical example of just how this might be used in talk.
There's a lot to do around here with Janet AWOL and a bunch of newly empty rooms.