The Collaborative International Dictionary of English v.0.48:

Prithee \Prith"ee\, interj.
   A corruption of pray thee; as, I prithee; generally used
   without I. --Shak.
   [1913 Webster]

         What was that scream for, I prithee?     --L'Estrange.
   [1913 Webster]

         Prithee, tell me, Dimple-chin.           --E. C.
                                                  Stedman.
   [1913 Webster]