Smith's Bible Dictionary

Rabmag (Jeremiah 39:3; Jeremiah 39:13) a title borne by Nergal-sharezer, probably identical with the king called by the Greeks Neriglissar. [NERGAL-SHAREZER] (it probably means chief of the magi; at all events it was "an office of great power and dignity at the Babylonian court, and probably gave its possessor special facilities for gaining the throne.")