GNU Collaborative International Dictionary of English

Found one definition

1.       
   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.]
   Audible expression of sorrow; lamentation; complaint; hence, a mournful song; a lament. Chaucer.“The Psalmist's mournful plaint.” Wordsworth.
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   An accusation or protest on account of an injury.
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   There are three just grounds of war with Spain: one of plaint, two upon defense.
   Bacon.
   
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   (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.
   
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