A.B.E. stands for "All but engaged" and refers to a couple who have been together seriously for a long time, and are expected to marry but have not officially announced engagement.
Similar to the A.B.D., "All but dissertation."
May be loosely substituted with the acronym, A.B.M.
, all but married.
Eunice and Thad are A.B.E.; they've been dating for over a year now and have talked about marriage. She's just waiting for him to pop the question.