The Collaborative International Dictionary of English v.0.48:

Covin \Cov"in\ (k?v"?n), n. [OF. covine, covaine, fr. covenir to
   agree. See Covenant.]
   1. (Law) A collusive agreement between two or more persons to
      prejudice a third.
      [1913 Webster]

   2. Deceit; fraud; artifice. [Obs.] --Chaucer.
      [1913 Webster]

Moby Thesaurus II by Grady Ward, 1.0:

47 Moby Thesaurus words for "covin":
   artifice, cabal, chicane, chicanery, collusion, complicity,
   complot, confederacy, connivance, connivery, conspiracy,
   contrivance, contriving, counterplot, deep-laid plot, dodgery,
   engineering, finagling, finesse, foul play, frame-up, game,
   intrigue, little game, machination, maneuvering, manipulation,
   pettifoggery, pettifogging, plot, plotting, practice, rigging,
   scheme, schemery, scheming, sharp practice, skulduggery, sleight,
   stratagem, supercherie, trick, trickery, underhand dealing,
   underplot, web of intrigue, wire-pulling



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

COVIN, fraud. A secret contrivance between two or more persons to defraud and

prejudice another of his rights. Co. Litt 357, b; Com. Dig. Covin, A; 1 Vin.
Abr. 473. Vide Collusion; Fraud.