The Collaborative International Dictionary of English v.0.48:

Cestuy \Ces"tuy\ or Cestui \Ces"tui\, pron. [Norm. F.] (Law)
   He; the one.
   [1913 Webster]

   Cestuy que trust[norm. F.], a person who has the equitable
      and beneficial interest in property, the legal interest in
      which is vested in a trustee. --Wharton.

   Cestuy que use[Norm. F.], a person for whose use land,
      etc., is granted to another.
      [1913 Webster]

Bouvier's Law Dictionary, Revised 6th Ed (1856):

CESTUI. He. This word is frequently used in composition as, cestui que
trust, cestui que vie, &c.