The Collaborative International Dictionary of English v.0.48:

Surcoat \Sur"coat`\, n. [OE. surcote, OF. surcote. See Sur-,
   and Coat, and cf. Overcoat.]
   1. A coat worn over the other garments; especially, the long
      and flowing garment of knights, worn over the armor, and
      frequently emblazoned with the arms of the wearer.
      [1913 Webster]

            A long surcoat of pers upon he had..  --Chaucer.
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            At night, or in the rain,
            He dons a surcoat which he doffs at morn. --Emerson.
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   2. A name given to the outer garment of either sex at
      different epochs of the Middle Ages.
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WordNet (r) 3.0 (2006):

surcoat
    n 1: a loose outer coat usually of rich material
    2: a tunic worn over a knight's armor