The Collaborative International Dictionary of English v.0.48:

Elfish \Elf"ish\, a.
   Of or relating to the elves; elflike; implike; weird;
   scarcely human; mischievous, as though caused by elves.
   "Elfish light." --Coleridge.
   [1913 Webster]

         The elfish intelligence that was so familiar an
         expression on her small physiognomy.     --Hawthorne.
   [1913 Webster]

WordNet (r) 3.0 (2006):

elfish
    adj 1: usually good-naturedly mischievous; "perpetrated a
           practical joke with elfin delight"; "elvish tricks" [syn:
           elfin, elfish, elvish]

Moby Thesaurus II by Grady Ward, 1.0:

37 Moby Thesaurus words for "elfish":
   arch, devilish, elfin, elflike, elvish, faery, fairy, fairyish,
   fairylike, fay, foolish, full of mischief, gnomelike, gnomish,
   high-spirited, impish, knavish, mischief-loving, mischievous,
   pixieish, playful, prankish, pranksome, pranky, puckish, roguish,
   scampish, scapegrace, sportive, sylphidine, sylphine, sylphish,
   sylphlike, sylphy, trickish, tricksy, waggish