The Collaborative International Dictionary of English v.0.48:

Plaint \Plaint\, n. [OE. plainte, pleynte, F. plainte, fr. L.
   plangere, planctum (plancta, fem. p. p.), to beat, beat the
   breast, lament. Cf. Complain, Plague, Plangent.]
   1. Audible expression of sorrow; lamentation; complaint;
      hence, a mournful song; a lament. --Chaucer."The
      Psalmist's mournful plaint." --Wordsworth.
      [1913 Webster]

   2. An accusation or protest on account of an injury.
      [1913 Webster]

            There are three just grounds of war with Spain: one
            of plaint, two upon defense.          --Bacon.
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   3. (Law) A private memorial tendered to a court, in which a
      person sets forth his cause of action; the exhibiting of
      an action in writing. --Blackstone.
      [1913 Webster]

WordNet (r) 3.0 (2006):

plaint
    n 1: (United Kingdom) a written statement of the grounds of
         complaint made to court of law asking for the grievance to
         be redressed
    2: a cry of sorrow and grief; "their pitiful laments could be
       heard throughout the ward" [syn: lament, lamentation,
       plaint, wail]