morally bad but in an attractive way: a devilish grin.

What is a devilish grin?

An "evil grin" is a specific type of emoticon or visual symbolizing a specific type of smile, a face with a wide grin and, typically, downturned eyebrows, signifying evil intent.

What does a devilish mean?

1 : resembling or befitting a devil: such as. a : evil, sinister. b : mischievous, roguish a devilish grin.