The Collaborative International Dictionary of English v.0.48:

Knavish \Knav"ish\, a.
   1. Like or characteristic of a knave[3]; given to knavery;
      trickish; fraudulent; dishonest; villainous; as, a knavish
      fellow, or a knavish trick. "Knavish politicians."
      --Macaulay.
      [1913 Webster]

   2. Mischievous; roguish; waggish; rascally.
      [1913 Webster]

            Cupid is knavish lad,
            Thus to make poor females mad.        --Shak.
      [1913 Webster]

WordNet (r) 3.0 (2006):

knavish
    adj 1: marked by skill in deception; "cunning men often pass for
           wise"; "deep political machinations"; "a foxy scheme"; "a
           slick evasive answer"; "sly as a fox"; "tricky Dick"; "a
           wily old attorney" [syn: crafty, cunning, dodgy,
           foxy, guileful, knavish, slick, sly, tricksy,
           tricky, wily]

Moby Thesaurus II by Grady Ward, 1.0:

74 Moby Thesaurus words for "knavish":
   abominable, arch, arrant, atrocious, bad, base, black,
   blackguardly, blamable, blameworthy, criminal, damnable, dark,
   deceitful, degraded, devilish, disgraceful, elfish, elvish, evil,
   execrable, flagitious, flagrant, foolish, foul, full of mischief,
   heinous, high-spirited, impish, improper, infamous, iniquitous,
   low, lying, mendacious, mischief-loving, mischievous, monstrous,
   naughty, nefarious, notorious, peccant, playful, prankish,
   pranksome, pranky, puckish, rank, rascally, recreant,
   reprehensible, reprobate, roguish, scampish, scandalous,
   scapegrace, scoundrelly, shameful, shifty, sinful, sportive,
   trickish, tricksy, unforgivable, unpardonable, unspeakable,
   untruthful, unworthy, vicious, vile, villainous, waggish, wicked,
   wrong