Proposed Canon
The exact meaning of the title is as of yet unclear; it may have to do with level, casting ability, rank among Hippiemancers, or some combination of these. Casting ability is at least partly involved, however, since Grand Abbies seem to be the best at - for instance - quieting battles.Erf-b1-p013
 Real World References
The title is possibly a reference to the title of abbot/abbess.
Mother Abagail is the personification of good in Stephen King's novel The Stand.