The Collaborative International Dictionary of English v.0.48:

Fay \Fay\, v. i. (Shipbuilding)
   To lie close together; to fit; to fadge; -- often with in,
   into, with, or together.
   [1913 Webster]

   Faying surface, that surface of an object which comes with
      another object to which it is fastened; -- said of plates,
      angle irons, etc., that are riveted together in shipwork.
      [1913 Webster]