The Collaborative International Dictionary of English v.0.48:

impish \imp"ish\ ([i^]mp"[i^]sh), a.
   Having the qualities, or showing the characteristics, of an
   imp; naughtily or annoyingly playful; as, teasing and
   worrying with impish laughter.

   Syn: elfish, elvish, implike, mischievous, pixilated,
        prankish, puckish.
        [1913 Webster + WordNet 1.5]

WordNet (r) 3.0 (2006):

impish
    adj 1: naughtily or annoyingly playful; "teasing and worrying
           with impish laughter"; "a wicked prank" [syn: arch,
           impish, implike, mischievous, pixilated,
           prankish, puckish, wicked]

Moby Thesaurus II by Grady Ward, 1.0:

22 Moby Thesaurus words for "impish":
   arch, devilish, elfish, elvish, foolish, full of mischief,
   high-spirited, knavish, mischief-loving, mischievous, playful,
   prankish, pranksome, pranky, puckish, roguish, scampish,
   scapegrace, sportive, trickish, tricksy, waggish