devilish

adjectives

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"
playful in an appealingly bold way
"a roguish grin"

adverbs

in a playfully devilish manner
"the socialists are further handicapped if they believe that capitalists are not only wicked but also devilishly clever"