Source : Webster's Revised Unabridged Dictionary (1913)

Habergeon \Ha*ber"ge*on\, n. [F. haubergeon a small hauberk,
   dim. of OF. hauberc, F. haubert. See Hauberk.]
   Properly, a short hauberk, but often used loosely for the
   hauberk. --Chaucer.

Source : WordNet®

habergeon
     n : (Middle Ages) a light sleeveless coat of chain mail worn
         under the hauberk