The Collaborative International Dictionary of English v.0.48:

Darrein \Dar"rein\, a. [OF. darrein, darrain, fr. an assumed LL.
   deretranus; L. de + retro back, backward.] (Law)
   Last; as, darrein continuance, the last continuance.
   [1913 Webster]

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

DARREIN. A corruption of the French word "dernier," the last. It is
sometimes used as, "darrein continuance," the last continuance. When any
matter has arisen in discharge of the defendant in action, he may take
advantage of it, provided he pleads itpuis darrein continuance; for if he
neglect to do so, he waives his right. Vide article darrein continuance.