The Collaborative International Dictionary of English v.0.48:

Expostulate \Ex*pos"tu*late\ (?; 135), v. i. [imp. & p. p.
   Expostulated; p. pr. & vb. n. Expostulating.] [L.
   expostulatus, p. p. of expostulare to demand vehemently; ex
   out + postulare to ask, require. See Postulate.]
   To reason earnestly with a person on some impropriety of his
   conduct, representing the wrong he has done or intends, and
   urging him to make redress or to desist; to remonstrate; --
   followed by with.
   [1913 Webster]

         Men expostulate with erring friends; they bring
         accusations against enemies who have done them a wrong.
                                                  --Jowett
                                                  (Thuc. ).

   Syn: To remonstrate; reason. See Remonstrate.
        [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

Expostulate \Ex*pos"tu*late\, v. t.
   To discuss; to examine. [Obs.]
   [1913 Webster]

         To expostulate
         What majesty should be, what duty is.    --Shak.
   [1913 Webster]

WordNet (r) 3.0 (2006):

expostulate
    v 1: reason with (somebody) for the purpose of dissuasion