The Collaborative International Dictionary of English v.0.48:

Perpender \Per*pend"er\, n. [F. parpaing, pierre parpaigne; of
   uncertain origin.] (Masonry)
   A large stone reaching through a wall so as to appear on both
   sides of it, and acting as a binder; -- called also
   perbend, perpend stone, and perpent stone.
   [1913 Webster]