Source : Webster's Revised Unabridged Dictionary (1913)

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

   2. Expressive of sorrow or melancholy; mournful; sad. ``The
      most plaintive ditty.'' --Landor. -- {Plain"tive*ly}, adv.
      -- {Plain"tive*ness}, n.

Source : WordNet®

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