>To have an evil plan
>To not be doing anything good for anybody
>To be inclined towards making something worse through a planned effort
It is generally associated with a mild playfulness but could just as easily point to nefarious and malicious behaviour.
Example
>The politician was up to no good and hence did not answer any questions relating to the allegations that he had made about the opposing party leader.
>I’m afraid she is up to no good spending all that time with her friends.