The Collaborative International Dictionary of English v.0.48:

Deceit \De*ceit"\, n. [OF. deceit, des[,c]ait, decept (cf.
   deceite, de[,c]oite), fr. L. deceptus deception, fr.
   decipere. See Deceive.]
   1. An attempt or disposition to deceive or lead into error;
      any declaration, artifice, or practice, which misleads
      another, or causes him to believe what is false; a
      contrivance to entrap; deception; a wily device; fraud.
      [1913 Webster]

            Making the ephah small and the shekel great, and
            falsifying the balances by deceit.    --Amos viii.
                                                  5.
      [1913 Webster]

            Friendly to man, far from deceit or guile. --Milton.
      [1913 Webster]

            Yet still we hug the dear deceit.     --N. Cotton.
      [1913 Webster]

   2. (Law) Any trick, collusion, contrivance, false
      representation, or underhand practice, used to defraud
      another. When injury is thereby effected, an action of
      deceit, as it called, lies for compensation.

   Syn: Deception; fraud; imposition; duplicity; trickery;
        guile; falsifying; double-dealing; stratagem. See
        Deception.
        [1913 Webster]

WordNet (r) 3.0 (2006):

deceit
    n 1: the quality of being fraudulent [syn: fraudulence,
         deceit]
    2: a misleading falsehood [syn: misrepresentation, deceit,
       deception]
    3: the act of deceiving [syn: deception, deceit,
       dissembling, dissimulation]

Moby Thesaurus II by Grady Ward, 1.0:

90 Moby Thesaurus words for "deceit":
   art, artful dodge, artfulness, artifice, blind, cheating, chicane,
   chicanery, con, con game, conspiracy, contrivance, coup, craft,
   craftiness, cunning, cute trick, deceitfulness, deception,
   defrauding, design, device, dishonesty, dissemblance,
   dissimulation, dodge, double-cross, double-dealing, duplicity,
   expedient, fakement, falseheartedness, falseness, feint, fetch,
   flam, flimflam, fraud, fraudulence, furtiveness, gambit, game,
   gimmick, grift, guile, gyp, hanky-panky, hoax, humbug, hypocrisy,
   indirection, insidiousness, intrigue, jugglery, knavery,
   little game, maneuver, misrepresentation, monkey business, move,
   overreaching, plot, ploy, racket, red herring, ruse, scam, scheme,
   sell, sham, shift, shiftiness, sleight, slyness, sneak attack,
   sneakiness, stratagem, strategy, subterfuge, surreptitiousness,
   swindle, tactic, trapping, treacherousness, treachery, trick,
   trickery, underhandedness, wile, wily device



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

DECEIT, tort. A fraudulent. misrepresentation or contrivance, by which one
man deceives another, who has no means of detecting the fraud, to the injury
and damage of the latter.
     2. Fraud, or the intention to deceive, is the very essence of this
injury, for if the party misrepresenting was himself mistaken, no blame can
attach to him. The representation must be made malo animo, but whether or
not the party is himself to gain by it, is wholly immaterial.
     3. Deceit may not only be by asserting a falsehood deliberately to the
injury of another as, that Paul is in flourishing circumstances, whereas he
is in truth insolvent; that Peter is an honest man, when he knew him to be
a, rogue; that property, real or personal, possesses certain qualities, or
belongs to the vendor, whereas he knew these things to be false; but by any
act or demeanor which would naturally impress the mind of a careful man with
a mistaken belief.
     4. Therefore, if one whose manufactures are of a superior quality,
distinguishes them by a particular mark, which facts are known to Peter, and
Paul counterfeits this work, and affixes them to articles of the same
description, but not made by such person, and sells them to Peter as goods
of such manufacture, this is a deceit.
     5. Again, the vendor having a knowledge of a defect in a commodity
which cannot be obvious to the buyer, does not disclose it, or, if apparent,
uses an artifice and conceals it, he has been guilty of a fraudulent
misrepresentation for there is an implied condition in every contract that
the parties to it act upon equal terms, and the seller is presumed to have
assured or represented to the vendee that he is not aware of any secret
deficiencies by which the commodity is impaired, and that he has no
advantage which himself does not possess.
     6. But in all these cases the party injured must have no means of
detecting the fraud, for if he has such means his ignorance will not avail
him in that case he becomes the willing dupe of the other's artifice, and
volenti non fit injuria. For example, if a horse is sold wanting an eye, and
the defect is visible to a common observer, the purchaser cannot be said to
be deceived,  for by inspection he might discover it, but if the blindness
is only discoverable by one experienced in such diseases, and the vendee is
an inexperienced person, it is a deceit, provided the seller knew of the
defect.
     7. The remedy for a deceit, unless the right of action has been
suspended or discharged, is by an action of trespass on the case. The old
writ of deceit was brought for acknowledging a fine, or the like, in another
name, and this being a perversion of law to an evil purpose, and a high
contempt, the act was laid contra pacem, and a fine imposed upon the
offender. See Bro. Abr. Disceit; Vin Abr. Disceit.
     8. When two or more persons unite in a deceit upon another, they may be
indicted for a conspiracy. (q.v.) Vide, generally, 2 Bouv. Inst. n. 2321-
29; Skin. 119; Sid. 375; 3 T. R. 52-65; 1 Lev. 247; 1 Strange, 583; D Roll.
Abr. 106; 7 Barr, Rep. 296; 11 Serg. & R. 309, 310; Com. Dig. Action upon
the case for a deceit; Chancery, 3 F 1 and 2; 3 M 1; 3 N 1; 4 D 3; 4 H 4; 4
L 1; 4 O 2; Covin; Justices of the Peace, B 30; Pleader, 2 H; 1 Vin. Ab.
560; 8 Vin. Ab. 490; Doct. Pl. 51; Dane's Ab. Index, h.t.; 1 Chit. Pr. 832
Ham. N. P. c. 2, s. 4; Ayl. Pand. 99 2 Day, 531; 12 Mass. 20; 3 Johns. 269;
6 Johns. 181; 2 Day, 205, 381; 4 Yeates, 522; 18 John. 395: 8 John. 23; 4
Bibb, 91; 1 N. & M. 197. Vide, also, articles Equality; Fraud; Lie.