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.
      [1913 Webster]

   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]

Moby Thesaurus II by Grady Ward, 1.0:

55 Moby Thesaurus words for "plaint":
   accusal, accusation, accusing, allegation, allegement, arraignment,
   bawl, bill of particulars, blame, bringing of charges,
   bringing to book, charge, complaint, count, cry, delation,
   denouncement, denunciation, dolorous tirade, groan, howl,
   impeachment, implication, imputation, indictment, information,
   innuendo, insinuation, jeremiad, keen, lament, lawsuit,
   laying of charges, moan, murmur, mutter, outcry, planctus,
   prosecution, reproach, scream, sob, suit, taxing, tirade,
   true bill, ululation, unspoken accusation, veiled accusation, wail,
   wail of woe, whimper, whine, yawp, yowl



Bouvier's Law Dictionary, Revised 6th Ed (1856):

PLAINT, Eng. law. The exhibiting of any action, real or personal, in
writing; the party making his plaint is called the plaintiff.