Source : Webster's Revised Unabridged Dictionary (1913)

Frock \Frock\, 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.

   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.

   {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.

Source : WordNet®

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