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):

plaintively
    adv 1: in a plaintive manner; "the last note of the song rang
           out plaintively"

Moby Thesaurus II by Grady Ward, 1.0:

19 Moby Thesaurus words for "plaintively":
   brokenheartedly, complainingly, dolefully, dolorously, fretfully,
   grieffully, grievously, groaningly, lamentingly, moanfully,
   mournfully, peevishly, petulantly, querulously, ruefully,
   sorrowfully, tearfully, wailfully, woefully