The Collaborative International Dictionary of English v.0.48:

Pshaw \Pshaw\, interj. [Of imitative origin.]
   Pish! pooch! -- an exclamation used as an expression of
   contempt, disdain, dislike, etc. [Written also psha.]
   [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

Pshaw \Pshaw\, v. i.
   To express disgust or contemptuous disapprobation, as by the
   exclamation " Pshaw!"
   [1913 Webster]

         The goodman used regularly to frown and pshaw wherever
         this topic was touched upon.             --Sir W.
                                                  Scott.
   [1913 Webster]