Source : Webster's Revised Unabridged Dictionary (1913)

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

   {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.