The Collaborative International Dictionary of English v.0.48:

Exergue \Ex*ergue"\, n. [F., fr. Gr. ? out + ? work; lit., out
   work, i. e., accessory work. See Work.] (Numis.)
   The small space beneath the base line of a subject engraved
   on a coin or medal. It usually contains the date, place,
   engraver's name, etc., or other subsidiary matter.
   --Fairholt.
   [1913 Webster]