The Collaborative International Dictionary of English v.0.48:

Prosy \Pros"y\, a. [Compar. Prosier; superl. Prosiest.]
   [1913 Webster]
   1. Of or pertaining to prose; like prose.
      [1913 Webster]

   2. Dull and tedious in discourse or writing; prosaic.
      [1913 Webster]

WordNet (r) 3.0 (2006):

prosy
    adj 1: lacking wit or imagination; "a pedestrian movie plot"
           [syn: pedestrian, prosaic, prosy, earthbound]

Moby Thesaurus II by Grady Ward, 1.0:

84 Moby Thesaurus words for "prosy":
   Attic, Spartan, arid, ascetic, austere, bald, bare, barren, candid,
   chaste, classic, classical, common, commonplace, direct, drab, dry,
   dull, earthbound, everyday, flat, frank, garden, garden-variety,
   homely, homespun, household, humdrum, infecund, infertile, insipid,
   lackluster, lean, lifeless, literal, lusterless, matter-of-fact,
   mundane, natural, neat, nondescript, open, ordinary, pedestrian,
   plain, plain-speaking, plain-spoken, poetryless, prosaic, prose,
   prosing, pure, pure and simple, rustic, severe, simple,
   simple-speaking, sober, spare, staid, stark, stolid,
   straightforward, stuffy, tiresome, unadorned, unaffected,
   unembellished, unfanciful, unideal, unidealistic, unimaginative,
   unimpassioned, uninspired, uninventive, unoriginal, unpoetic,
   unpoetical, unromantic, unromanticized, unvarnished, vapid,
   workaday, workday