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):

plaintive
    adj 1: expressing sorrow [syn: mournful, plaintive]

Moby Thesaurus II by Grady Ward, 1.0:

44 Moby Thesaurus words for "plaintive":
   Jeremianic, aggrieved, anguished, careworn, complaining, doleful,
   dolorous, dumb with grief, faultfinding, fretful, grief-stricken,
   griefful, grieved, grievous, howling, in grief, lamentable,
   lamenting, lamentive, lugubrious, moanful, mournful, peevish,
   petulant, piteous, pitiful, plangent, plunged in grief, puling,
   querulous, rueful, sad, saddening, sorrowed, sorrowful, sorrowing,
   tearful, ululant, wailful, wailing, whimpering, whining, whiny,
   woeful