Source : Webster's Revised Unabridged Dictionary (1913)

Tabard \Tab"ard\, n. [OE. tabard, tabart; cf. Sp. & Pg. tabardo,
   It. tabarro, W. tabar, LGr. ?, LL. tabardum.]
   A sort of tunic or mantle formerly worn for protection from
   the weather. When worn over the armor it was commonly
   emblazoned with the arms of the wearer, and from this the
   name was given to the garment adopted for heralds. [Spelt
   also {taberd}.]

         In a tabard he [the Plowman] rode upon a mare.
                                                  --Chaucer.

Source : WordNet®

tabard
     n : a short sleeveless outer tunic emblazoned with a coat of
         arms; worn by a knight over his armor or by a herald