A.B.M. stands for "All but married." Similar to A.B.E.
, A.B.M. may denote that a couple is engaged, but not yet married. In particular, it may denote a rather long engagement period, but not always. See example (1)
Other times, it may be used as a loose synonym for A.B.E.
and may or may not denote behavior between a couple that is more serious than expected at an early stage of dating. See example (2)
Similar to the academic lingo, A.B.D., All but dissertation.