The Collaborative International Dictionary of English v.0.48:

Hack \Hack\, a.
   Hackneyed; hired; mercenary. --Wakefield.
   [1913 Webster]

   Hack writer, a hack; one who writes for hire. "A vulgar
      hack writer." --Macaulay.
      [1913 Webster]

WordNet (r) 3.0 (2006):

hack writer
    n 1: a mediocre and disdained writer [syn: hack, hack
         writer, literary hack]