What does "fay" mean?

Additional senses

1. 2.Faith; as, by my fay. [Obs.] Chaucer.
2. 3.To fit; to join; to unite closely, as two pieces of wood, so as to make the surface fit together.
3. 4.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.