Source : Webster's Revised Unabridged Dictionary (1913)

Cassock \Cas"sock\, n. [F. casaque, fr. It. casacca, perh. fr.
   L. casa cottage, in It., house; or of Slavic origin.]
   1. A long outer garment formerly worn by men and women, as
      well as by soldiers as part of their uniform.

   2. (Eccl.) A garment resembling a long frock coat worn by the
      clergy of certain churches when officiating, and by others
      as the usually outer garment.

Source : WordNet®

cassock
     n : a black garment reaching down to the ankles; worn by priests
         or choristers