The Collaborative International Dictionary of English v.0.48:

Knave \Knave\ (n[=a]v), n. [OE., boy, servant, knave, AS. cnafa
   boy, youth; cf. AS. cnapa boy, youth, D. knaap, G. knabe boy,
   knappe esquire, Icel. knapi, Sw. knape esquire, kn[aum]fvel
   knave.]
   1. A boy; especially, a boy servant. [Obs.] --Wyclif.
      Chaucer.
      [1913 Webster]

            O murderous slumber,
            Lay'st thou thy leaden mace upon my boy
            That plays thee music ? Gentle knave, good night.
                                                  --Shak.
      [1913 Webster]

   2. Any male servant; a menial. [Obs.] --Chaucer.
      [1913 Webster]

            He's but Fortune's knave,
            A minister of her will.               --Shak.
      [1913 Webster]

   3. A tricky, deceitful fellow; a dishonest person; a rogue; a
      villain. "A pair of crafty knaves." --Shak.
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            In defiance of demonstration, knaves will continue
            to proselyte fools.                   --Ames.
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   Note: "How many serving lads must have been unfaithful and
         dishonest before knave -- which meant at first no more
         than boy -- acquired the meaning which it has now !"
         --Trench.
         [1913 Webster]

   4. A playing card marked with the figure of a servant or
      soldier; a jack; as, the knave of hearts.
      [1913 Webster]

   Knave child, a male child. [Obs.] --Chaucer.

   Syn: Villain; cheat; rascal; rogue; scoundrel; miscreant.
        [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

Varlet \Var"let\, n. [OF. varlet, vaslet, vallet, servant, young
   man, young noble, dim. of vassal. See Vassal, and cf.
   Valet.]
   [1913 Webster]
   1. A servant, especially to a knight; an attendant; a valet;
      a footman. [Obs.] --Spenser. Tusser.
      [1913 Webster]

   2. Hence, a low fellow; a scoundrel; a rascal; as, an
      impudent varlet.
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            What a brazen-faced varlet art thou ! --Shak.
      [1913 Webster]

   3. In a pack of playing cards, the court card now called the
      knave, or jack. [Obs.]
      [1913 Webster]

WordNet (r) 3.0 (2006):

knave
    n 1: a deceitful and unreliable scoundrel [syn: rogue,
         knave, rascal, rapscallion, scalawag, scallywag,
         varlet]
    2: one of four face cards in a deck bearing a picture of a young
       prince [syn: jack, knave]

Moby Thesaurus II by Grady Ward, 1.0:

77 Moby Thesaurus words for "knave":
   ace, bad boy, bastard, best bower, blackguard, booger, bower,
   buffoon, bugger, cards, clubs, cutup, deck, deuce, devil, diamonds,
   dummy, elf, enfant terrible, face cards, flush, full house,
   funmaker, hand, hearts, heel, hood, hoodlum, hooligan, imp, jack,
   joker, jokester, king, left bower, little devil, little monkey,
   little rascal, lowlife, minx, mischief, mischief-maker, miscreant,
   pack, pair, picture cards, pixie, playing cards, practical joker,
   prankster, precious rascal, puck, queen, rapscallion, rascal,
   rogue, round, rowdy, royal flush, rubber, ruff, ruffian, scalawag,
   scamp, scapegrace, scoundrel, shyster, singleton, sneak, spades,
   spalpeen, straight, trey, trick, trump, villain, wag



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

KNAVE. A false, dishonest, or deceitful person. This signification of the
word has arisen by a long perversion of its original meaning.
     2. To call a man a knave has been held to be actionable. 1 Rolle's Ab.
52; 1 Freem. 277.,