The Collaborative International Dictionary of English v.0.48:

Codpiece \Cod"piece`\, n. [Cod, n., ? + piece.]
   A part of male dress in front of the breeches, formerly made
   very conspicuous. --Shak. --Fosbroke.
   [1913 Webster]

WordNet (r) 3.0 (2006):

codpiece
    n 1: (15th-16th century) a flap for the crotch of men's tight-
         fitting breeches