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.
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            O murderous slumber,
            Lay'st thou thy leaden mace upon my boy
            That plays thee music ? Gentle knave, good night.
                                                  --Shak.
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   2. Any male servant; a menial. [Obs.] --Chaucer.
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            He's but Fortune's knave,
            A minister of her will.               --Shak.
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   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.
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   4. A playing card marked with the figure of a servant or
      soldier; a jack; as, the knave of hearts.
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   Knave child, a male child. [Obs.] --Chaucer.

   Syn: Villain; cheat; rascal; rogue; scoundrel; miscreant.
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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.]
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   1. A servant, especially to a knight; an attendant; a valet;
      a footman. [Obs.] --Spenser. Tusser.
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   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.
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   3. In a pack of playing cards, the court card now called the
      knave, or jack. [Obs.]
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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]