The Collaborative International Dictionary of English v.0.48:

Chin \Chin\ (ch[i^]n), n. [AS. cin, akin to OS. kin, G kinn,
   Icel. kinn, cheek, Dan. & Sw. kind, L. gena, Gr. ?; cf.
   Skr. hanu. [root]232.]
   1. The lower extremity of the face below the mouth; the point
      of the under jaw.
      [1913 Webster]

   2. (Zool.) The exterior or under surface embraced between the
      branches of the lower jaw bone, in birds.
      [1913 Webster]