Source : Webster's Revised Unabridged Dictionary (1913)

Devilish \Dev"il*ish\, a.
   1. Resembling, characteristic of, or pertaining to, the
      devil; diabolical; wicked in the extreme. ``Devilish
      wickedness.'' --Sir P. Sidney.

            This wisdom descendeth not from above, but is
            earthly, sensual, devilish.           --James iii.
                                                  15.

   2. Extreme; excessive. [Colloq.] --Dryden.

   Syn: Diabolical; infernal; hellish; satanic; wicked;
        malicious; detestable; destructive. -- {Dev"il*ish*ly},
        adv. -- {Dev"il*ish*ness}, n.

Source : WordNet®

devilish
     adj 1: showing the cunning or ingenuity or wickedness typical of a
            devil; "devilish schemes"; "the cold calculation and
            diabolic art of some statesmen"; "the diabolical
            expression on his face"; "a mephistophelian glint in
            his eye" [syn: {diabolic}, {diabolical}, {mephistophelian},
             {mephistophelean}]
     2: playful in an appealingly bold way; "a roguish grin" [syn: {rascally},
         {roguish}]

devilish
     adv : in a playfully devilish manner; "the socialists are further
           handicapped if they believe that capitalists are not
           only wicked but also devilishly clever" [syn: {devilishly}]