The Collaborative International Dictionary of English v.0.48:

Explat \Ex*plat"\, Explate \Ex*plate"\, v. t. [Pref. ex- + plat
   or plait.]
   To explain; to unfold. [Obs.]
   [1913 Webster]

         Like Solon's self explatest the knotty laws. --B.
                                                  Jonson.
   [1913 Webster]