The Collaborative International Dictionary of English v.0.48:

Habergeon \Ha*ber"ge*on\ (h[.a]*b[~e]r"j[-e]*[o^]n or
   h[a^]b"[~e]r*j[u^]n), 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.
   [1913 Webster]

WordNet (r) 3.0 (2006):

habergeon
    n 1: (Middle Ages) a light sleeveless coat of chain mail worn
         under the hauberk

Easton's 1897 Bible Dictionary:

Habergeon
   an Old English word for breastplate. In Job 41:26 (Heb. shiryah)
   it is properly a "coat of mail;" the Revised Version has
   "pointed shaft." In Ex. 28:32, 39:23, it denotes a military
   garment strongly and thickly woven and covered with mail round
   the neck and breast. Such linen corselets have been found in
   Egypt. The word used in these verses is _tahra_, which is of
   Egyptian origin. The Revised Version, however, renders it by
   "coat of mail." (See ARMOUR.)