The Collaborative International Dictionary of English v.0.48:

Banc \Banc\, Bancus \Ban"cus\, Bank \Bank\, n. [OF. banc, LL.
   bancus. See Bank, n.]
   A bench; a high seat, or seat of distinction or judgment; a
   tribunal or court.
   [1913 Webster]

   In banc, In banco (the ablative of bancus), In bank, in
      full court, or with full judicial authority; as, sittings
      in banc (distinguished from sittings at nisi prius).
      [1913 Webster]

Bouvier's Law Dictionary, Revised 6th Ed (1856):

BANC or BANK. The first of these is a French word signifying bench,
pronounced improperly bank. 1. The seat of judgment, as banc le roy, the
king's bench banc le common pleas, the bench of common pleas.
     2. The meeting of all the judges or such as may form a quorum, as, the
court sit in banc. Cowell's Interp.