The Collaborative International Dictionary of English v.0.48:

Plaintive \Plain"tive\, a. [F. plaintif. See Plaintiff, n.]
   1. Repining; complaining; lamenting. --Dryden.
      [1913 Webster]

   2. Expressive of sorrow or melancholy; mournful; sad. "The
      most plaintive ditty." --Landor.
      [1913 Webster] -- Plain"tive*ly, adv. --
      Plain"tive*ness, n.
      [1913 Webster]

WordNet (r) 3.0 (2006):

plaintiveness
    n 1: expressing sorrowfulness