The Collaborative International Dictionary of English v.0.48:

Frock \Frock\ (fr[o^]k), n. [F. froc a monk's cowl, coat,
   garment, LL. frocus, froccus, flocus, floccus, fr. L. floccus
   a flock of wool; hence orig., a flocky cloth or garment; cf.
   L. flaccus flabby, E. flaccid.]
   1. A loose outer garment; especially, a gown forming a part
      of European modern costume for women and children; also, a
      coarse shirtlike garment worn by some workmen over their
      other clothes; a smock frock; as, a marketman's frock.
      [1913 Webster]

   2. A coarse gown worn by monks or friars, and supposed to
      take the place of all, or nearly all, other garments. It
      has a hood which can be drawn over the head at pleasure,
      and is girded by a cord.
      [1913 Webster]

   Frock coat, a body coat for men, usually double-breasted,
      the skirts not being in one piece with the body, but sewed
      on so as to be somewhat full.

   Smock frock. See in the Vocabulary.
      [1913 Webster]

WordNet (r) 3.0 (2006):

frock coat
    n 1: a man's coat having knee-length skirts front and back; worn
         in the 19th century