From ErfWiki

Proposed Canon

A title among Hippiemancers, held by Janis and implying great skill with Hippiemancy.

Given that Janis is "a Grand Abbie"^Erf-b1-p141 it is highly unlikely that Grand Abbie is a unique title with only a single holder at once.

Speculation

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 'Abbie' of the title probably refers to Abbie Hoffman.

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.