The Collaborative International Dictionary of English v.0.48:

Tresayle \Tres"ayle`\ (tr[e^]s"[=a]l`), n. [F. trisa["i]eul,
   from L. tris, tres, three + F. a["i]eul grandfather. Cf.
   Besaiel, and see Ayle.]
   A grandfather's grandfather. [Obs.]
   [1913 Webster]

   Writ of tresayle (O. Eng. Law), a writ which lay for a man
      claiming as heir to his grandfather's grandfather, to
      recover lands of which he had been deprived by an
      abatement happening on the ancestor's death. --Mozley & W.
      [1913 Webster]