rascally

Like or characteristic of a rascal; base; mean; trickish; scampish: used of persons or things with much latitude, often with slight, meaning.

Like a rascal; trickish or dishonest; base; worthless; -- often in humorous disparagement, without implication of dishonesty.

Like a rascal.

In the manner of a rascal.

Lacking principles or scruples.