Of an ecclesiastic: not wearing a mitre; not having episcopal rank or authority; (especially of a bishop) deprived of office, dismissed. Hence (of a non-ecclesiastic): having authority or influence compared to that of a bishop.
specifically of an abbot: not entitled to wear a mitre.
Late 17th century; earliest use found in Randle Holme (1627–1700), herald painter. From un- + mitred.